Palestinian refugees

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Palestinian_refugees

Palestinian refugees are citizens of Mandatory Palestine, and their descendants, who fled or were expelled from their country over the course of the 1947–49 Palestine war (1948 Palestinian exodus) and the Six-Day War (1967 Palestinian exodus). Most Palestinian refugees live in or near 68 Palestinian refugee camps across Jordan, Lebanon, Syria, the West Bank and the Gaza Strip. In 2019 more than 5.6 million Palestinian refugees were registered with the United Nations.

In 1949, the United Nations Relief and Works Agency for Palestine Refugees in the Near East (UNRWA) defined Palestinian refugees to refer to the original "Palestine refugees" as well as their patrilineal descendants. However, UNRWA's assistance is limited to Palestine refugees residing in UNRWA's areas of operation in the Palestinian Territories, Lebanon, Jordan and Syria.

As of 2019, more than 5.6 million Palestinians were registered with UNRWA as refugees, of which more than 1.5 million live in UNRWA-run camps. The term "Palestine refugee" does not include internally displaced Palestinians, who became Israeli citizens, or displaced Palestinian Jews. According to some estimates, as many as 1,050,000–1,380,000 people, who descend from displaced people of Mandatory Palestine are not registered under UNRWA or UNHCR mandates.

During the 1948 Palestine War, around 700,000 Palestinian Arabs or 85% of the total population in what became Israel fled or were expelled from their homes, to the West Bank, the Gaza Strip, and to the countries of Lebanon, Syria and Jordan. They, and their descendants who are also entitled to registration, are assisted by UNWRA in 59 registered camps, ten of which were established in the aftermath of the Six-Day War in 1967 to cope with the new wave of displaced Palestinians. They are also the world's oldest unsettled refugee population, having been under the ongoing governance of Arab states following the 1948 Arab–Israeli War, the refugee populations of the West Bank under Israeli governance since the Six-Day War and Palestinian administration since 1994, and the Gaza Strip administered by the Islamic Resistance Movement (Hamas) since 2007.

Today, the largest number of refugees, over 2,000,000, live in Jordan, where by 2009 over 90% of UNWRA-registered Palestinian refugees had acquired full citizenship rights. This figure consists almost exclusively of West Bank-descended Palestinians; however, as of December 2021, Palestinians with roots in the Gaza Strip are also still kept in legal limbo. In 2021, Jordanian politician Jawad Anani estimated that roughly 50% of Jordan's population had West Bank-Palestinian roots. Another approximately 2,000,000 refugees live in the West Bank and Gaza Strip, under Israeli occupation and blockade. Approximately 500,000 refugees live in each of Syria and Lebanon respectively, albeit under very different circumstances. While Palestinian refugees in Syria maintained their stateless status, the Syrian government afforded them the same economic and social rights enjoyed by Syrian citizens; they are also drafted into the Armed Forces despite not being citizens. Citizenship or legal residency in some host countries is denied, most notably for the Palestinian refugees in Lebanon, where the absorption of Palestinians would upset a delicate confessional balance. For the refugees themselves, these situations mean they have reduced rights: no right to vote, limited property rights and access to social services, among other things.

On 11 December 1948, the General Assembly of the United Nations (UNGA) adopted Resolution 194 which affirmed the Palestinians right to return to their homes.

Definitions

UNRWA

Palestinian refugees in Aida Refugee Camp, Bethlehem, 1956.

The United Nations Relief and Works Agency (UNRWA) is an organ of the United Nations created exclusively for the purpose of aiding those displaced by the Arab–Israeli conflict, with an annual budget of approximately $600 million. It defines a "Palestine refugee" as a person "whose normal place of residence was Mandatory Palestine between June 1946 and May 1948, who lost both their homes and means of livelihood as a result of the 1948 Arab–Israeli conflict". The Six-Day War of 1967 generated a new wave of Palestinian refugees who could not be included in the original UNRWA definition. From 1991, the UN General Assembly has adopted an annual resolution allowing the 1967 refugees within the UNRWA mandate. UNRWA aids all "those living in its area of operations who meet its working definition, who are registered with the Agency and who need assistance" and those who first became refugees as a result of the Six-Day War, regardless whether they reside in areas designated as Palestine refugee camps or in other permanent communities.

A Palestine refugee camp is "a plot of land placed at the disposal of UNRWA by the host government to accommodate Palestine refugees and to set up facilities to cater to their needs". About 1.4 million of registered Palestine refugees, approximately one-third, live in the 58 UNRWA-recognised refugee camps in Jordan, Lebanon, Syria, the Gaza Strip and the West Bank. The UNRWA definition does not cover final status.

Registered descendants of UNRWA Palestine refugees, like "Nansen passport" and "Certificate of Eligibility" holders (the documents issued to those displaced by World War II) or like UNHCR refugees, inherit the same Palestine refugee status as their male parent. According to UNRWA, "The descendants of Palestine refugee males, including adopted children, are also eligible for registration."

The UNHCR had counted 90,000 refugees by 2014.

Palestinian definitions

Palestinians make several distinctions relating to Palestinian refugees. The 1948 refugees and their descendants are broadly defined as "refugees" (laji'un). The Palestine Liberation Organization (PLO), especially those who have returned and form part of the PNA, but also Palestinian refugee camp residents in Lebanon, repudiate this term, since it implies being a passive victim, and prefer the autonym of 'returnees' (a'idun). Those who left since 1967, and their descendants, are called nazihun or "displaced persons", though many may also descend from the 1948 group.

Origin of the Palestine refugees

Most Palestinian refugees have retained their refugee status and continue to reside in refugee camps, including within the State of Palestine in the West Bank and in the Gaza Strip. Their descendants form a sizable portion of the Palestinian diaspora.

Palestinian refugees from the 1948 Palestine War

Main articles: 1948 Palestinian exodus, Causes of the 1948 Palestinian exodus, and Palestinian Exodus 1949 to 1956

During the 1948 Palestine War, some 700,000 Palestinian Arabs or 85% of the Palestinian Arab population of territories that became Israel fled or were expelled from their homes. Some 30,000 to 50,000 were alive by 2012.

The causes and responsibilities of the exodus are a matter of controversy among historians and commentators of the conflict. While historians agree on most of the events of the period, there remains disagreement as to whether the exodus was the result of a plan designed before or during the war or was an unintended consequence of the war. According to historian Benny Morris, the expulsion was planned and encouraged by the Zionist leadership.

According to Morris, between December 1947 and March 1948, around 100,000 Palestine Arabs fled. Among them were many from the higher and middle classes from the cities, who left voluntarily, expecting to return when the Arab states won the war and took control of the country. When the Haganah and then the emerging Israeli army (Israel Defense Forces or IDF) went on the defensive, between April and July, a further 250,000 to 300,000 Palestinian Arabs left or were expelled, mainly from the towns of Haifa, Tiberias, Beit-Shean, Safed, Jaffa and Acre, which lost more than 90 percent of their Arab inhabitants. Expulsions took place in many towns and villages, particularly along the Tel Aviv–Jerusalem road and in Eastern Galilee. About 50,000–70,000 inhabitants of Lydda and Ramle were expelled towards Ramallah by the IDF during Operation Danny, and most others during operations of the IDF in its rear areas. During Operation Dekel, the Arabs of Nazareth and South Galilee were allowed to remain in their homes. Today they form the core of the Arab Israeli population. From October to November 1948, the IDF launched Operation Yoav to remove Egyptian forces from the Negev and Operation Hiram to remove the Arab Liberation Army from North Galilee during which at least nine events named massacres of Arabs were carried out by IDF soldiers. These events generated an exodus of 200,000 to 220,000 Palestinian Arabs. Here, Arabs fled fearing atrocities or were expelled if they had not fled. After the war, from 1948 to 1950, the IDF resettled around 30,000 to 40,000 Arabs from the borderlands of the new Israeli state.

Palestinian refugees from Six-Day War

Main article: 1967 Palestinian exodus

As a result of the Six-Day War, around 280,000 to 325,000 Palestinians fled or were expelled from the territories won in the Six-Day War by Israel, including the demolished Palestinian villages of Imwas, Yalo, Bayt Nuba, Surit, Beit Awwa, Beit Mirsem, Shuyukh, Jiftlik, Agarith and Huseirat, and the "emptying" of the refugee camps of Aqabat Jabr and Ein as-Sultan.

Palestinian exodus from Kuwait (Gulf War)

Main article: Palestinian exodus from Kuwait (Gulf War)

The Palestinian exodus from Kuwait took place during and after the Gulf War. During the Gulf War, more than 200,000 Palestinians voluntarily fled Kuwait during the Iraqi occupation of Kuwait due to harassment and intimidation by Iraqi security forces, in addition to getting fired from work by Iraqi authority figures in Kuwait. After the Gulf War, Kuwaiti authorities forcibly pressured nearly 200,000 Palestinians to leave Kuwait in 1991. Kuwait's policy, which led to this exodus, was a response to alignment of Palestinian leader Yasser Arafat and the Palestine Liberation Organization (PLO) with the dictator Saddam Hussein, who had earlier invaded Kuwait.

Prior to the Gulf War, Palestinians numbered 400,000 out of Kuwait's population of 2.2 million. The Palestinians who fled Kuwait were Jordanian citizens. In 2013, there were 280,000 Jordanian citizens of Palestinian origin in Kuwait. In 2012, 80,000 Palestinians (without Jordanian citizenship) lived in Kuwait. In total, there are 360,000 Palestinians in Kuwait as of 2012–2013.

Palestinian refugees as part of the Syrian refugee crisis

Main article: Refugees of the Syrian Civil War

Many Syrian Palestinians were displaced as a result of the Syrian Civil War starting in 2011. By October 2013, 235,000 Palestinians had been displaced within Syria itself, and 60,000 (alongside 2.2 million Syrians) had fled the country. By March 2019, the UHCR estimated that 120,000 Palestine refugees had fled Syria since 2011, primarily to Lebanon and Jordan, but also Turkey and further afield.

There were reports that Jordan and Lebanon have turned away Palestinian refugees attempting to flee the humanitarian crises in Syria. By 2013, Jordan had absorbed 126,000 Syrian refugees, but Palestinians fleeing Syria are placed in a separate refugee camp, under stricter conditions and are banned from entering Jordanian cities.

Palestinian refugees from Syria are also immigrating to Europe seeking asylum, especially to Sweden, which has offered asylum to any Syrian refugees that manage to reach its territory, albeit with some conditions. Many do so by finding their way to Egypt and making the journey by sea. In October 2013, the PFLP-GC claimed that some 23,000 Palestinian refugees from the Yarmouk Camp had immigrated to Sweden alone.

Refugee statistics

Further information: Palestinian refugee camps

 

Destroyed house in the Jabalia refugee camp, Gaza–Israel conflict, December 2012

The number of Palestine refugees varies depending on the source. For 1948–49 refugees, for example, the Israeli government suggests a number as low as 520,000 as opposed to 850,000 by their Palestinian counterparts. As of January 2015, UNRWA cites 5,149,742 registered refugees in total, of whom 1,603,018 are registered in camps.

District	Number of depopulated villages	Number of refugees in 1948	Number of refugees in 2000
Beersheba	88	90,507	590,231
Beisan	31	19,602	127,832
Jenin	6	4,005	26,118
Haifa	59	121,196	790,365
Hebron	16	22,991	149,933
Ramle	64	97,405	635,215
Safad	78	52,248	340,729
Tiberias	26	28,872	188,285
Tulkarm	18	11,032	71,944
Acre	30	47,038	306,753
Gaza	46	79,947	521,360
Jerusalem	39	97,950	638,769
Nazareth	5	8,746	57,036
Jaffa	25	123,227	803,610
Total	531	804,766	5,248,185

The number of UNRWA registered Palestine refugees by country or territory in January 2015 were as follows:

Jordan	2,117,361
Gaza Strip	1,276,929
West Bank	774,167
Syria	528,616
Lebanon	452,669
Total	5,149,742

Pardes Hana Immigrant Camp of Jewish refugees, 1956.

In the context of the Arab–Israeli conflict, Jewish refugees were initially resettled in refugee camps known variously as Immigrant camps, Ma'abarot, and "development towns" prior to absorption into mainstream Israeli society. Conversely, many Palestinian refugees remain settled in Palestinian refugee camps, while others have been absorbed into Jordanian society or the Palestinian territories. Since 1948, the sovereign State of Israel has guaranteed asylum and citizenship to Jewish refugees, while the self-declared State of Palestine remains unable to absorb the Palestinian refugees, due to lack of de facto sovereignty over its claimed territories.

Gaza Strip

2018 Gaza border protests, Bureij refugee camp in Gaza

As of January 2015, the Gaza Strip has 8 UNRWA refugee camps with 560,964 Palestinian refugees, and 1,276,929 registered refugees in total, out of a population of 1,816,379.

West Bank

As of January 2015, the West Bank has 19 UNRWA refugee camps with 228,560 Palestinian refugees, and 774,167 registered refugees in total, out of a population of 2,345,107.

Jordan

See also: Palestinians in Jordan

"More than 2 million registered Palestine refugees live in Jordan. Most Palestine refugees in Jordan, but not all, have full citizenship", following Jordan's annexation and occupation of the West Bank. The percentage of Palestinian refugees living in refugee camps to those who settled outside the camps is the lowest of all UNRWA fields of operations. Palestine refugees are allowed access to public services and healthcare, as a result, refugee camps are becoming more like poor city suburbs than refugee camps. Most Palestine refugees moved out of the camps to other parts of the country and the number of people registered in refugee camps as of January 2015 is 385,418, who live in ten refugee camps. This caused UNRWA to reduce the budget allocated to Palestine refugee camps in Jordan. Former UNRWA chief-attorney James G. Lindsay wrote in 2009: "In Jordan, where 2 million Palestinian refugees live, all but 167,000 have citizenship, and are fully eligible for government services including education and health care." Lindsay suggests that eliminating services to refugees whose needs are subsidized by Jordan "would reduce the refugee list by 40%".

Palestinians who moved from the West Bank (whether refugees or not) to Jordan, are issued yellow-ID cards to distinguish them from the Palestinians of the "official 10 refugee camps" in Jordan. From 1988 to 2012, thousands of those yellow-ID card Palestinians had their Jordanian citizenship revoked. Human Rights Watch estimated that about 2,700 Palestinians were stripped of Jordanian nationality between 2004 and 2008. In 2012, the Jordanian government promised to stop revoking the citizenship of some Palestinians, and restored citizenship to 4,500 Palestinians who had previously lost it.

Lebanon

See also: Palestinians in Lebanon, Karantina massacre, Tel al-Zaatar massacre, and Sabra and Shatila massacre

 

Shatila refugee camp on the outskirts of Beirut in May 2019

 

Entrance to the Bourj el-Barajneh refugee camp in southern Beirut

100,000 Palestinians fled to Lebanon because of the 1948 Arab–Israeli War and were not allowed to return. As of January 2015, there were 452,669 registered refugees in Lebanon.

In a 2007 study, Amnesty International denounced the "appalling social and economic condition" of Palestinians in Lebanon. Until 2005, Palestinians were forbidden to work in over 70 jobs because they do not have Lebanese citizenship, but this was later reduced to around 20 as of 2007 after liberalization laws. In 2010, Palestinians were granted the same rights to work as other foreigners in the country.

Lebanon gave citizenship to about 50,000 Christian Palestinian refugees during the 1950s and 1960s. In the mid-1990s, about 60,000 Shiite Muslim refugees were granted citizenship. This caused a protest from Maronite authorities, leading to citizenship being given to all Christian refugees who were not already citizens.

In the 2010s, many Palestinian refugees in Lebanon began immigrating to Europe, both legally and illegally, as part of the European migrant crisis, due to a deterioration in living conditions there as part of the Syrian civil war. In December 2015, sources told Al Jazeera that thousands of Palestinians were fleeing to Europe by way of Turkey, with about 4,000 having fled the Ain al-Hilweh camp alone in recent months. Many were reaching Germany, with others going to Russia, Sweden, Belgium, and Norway. A census completed in January 2018 found that only around 175,000 Palestinian refugees were living in Lebanon, as opposed to previous UNRWA figures which put the number at between 400,000 and 500,000, as well as other estimates that placed the number between 260,000 and 280,000.

According to writer and researcher Mudar Zahran, a Jordanian of Palestinian heritage, the media chose to deliberately ignore the conditions of the Palestinians living in Lebanese refugee camps, and that the "tendency to blame Israel for everything" has provided Arab leaders with an excuse to deliberately ignore the human rights of the Palestinian in their countries.

Syria

See also: Palestinians in Syria

Syria had 528,616 registered Palestinian refugees in January 2015. There were 9 UNRWA refugee camps with 178,666 official Palestinian refugees.

As a result of the Syrian civil war, large numbers of Palestinian refugees fled Syria to Europe as part of the European migrant crisis, and to other Arab countries. In September 2015, a Palestinian official said that only 200,000 Palestinian refugees were left in Syria, with 100,000 Palestinian refugees from Syria in Europe and the remainder in other Arab countries.

Saudi Arabia

An estimated 240,000 Palestinians are living in Saudi Arabia. Palestinians are the sole foreign group that cannot benefit from a 2004 law passed by Saudi Arabia's Council of Ministers, which entitles expatriates of all nationalities who have resided in the kingdom for ten years to apply for citizenship.

Iraq

Main article: Palestinians in Iraq

There were 34,000 Palestinian refugees living in Iraq prior to the Iraq War. In the aftermath of the war, the majority fled to neighboring Jordan and Syria, or were killed. Thousands lived as internally displaced persons within Iraq or were stranded in camps along Iraq's borders with Jordan and Syria, as no country in the region would accept them, and lived in temporary camps along the no man's land in the border zones.

Other countries

India agreed to take in 165 refugees, with the first group arriving in March 2006. Generally, they were unable to find work in India as they spoke only Arabic though some found employment with UNHCR's non-governmental partners. All of them were provided with free access to public hospitals. Of the 165 refugees, 137 of them later found clearance for resettlement in Sweden. In November 2006, 54 were granted asylum in Canada, and in 2007, some 200 were accepted for resettlement in Sweden and Iceland, and Brazil agreed to take 100.

In 2009, significant numbers of these refugees were allowed to resettle abroad. More than 1,000 were accepted by various countries in Europe and South America, and an additional 1,350 were cleared for resettlement in the United States. Another 68 were allowed to resettle in Australia. However, the majority of Palestine refugees strongly oppose resettlement and much rather want to return.

Positions

Main article: Palestinian right of return

On 11 December 1948 the United Nations General Assembly discussed Bernadotte's report and passed a resolution: "that refugees wishing to return to their homes and live at peace with their neighbour should be permitted to do so at the earliest practicable date. " This General Assembly article 11 of Resolution 194 has been annually re-affirmed.

Israeli views

The Jewish Agency promised to the UN before 1948 that Palestinian Arabs would become full citizens of the State of Israel, and the Israeli declaration of independence invited the Arab inhabitants of Israel to "full and equal citizenship". In practice, Israel does not grant citizenship to the refugees, as it does to those Arabs who continue to reside in its borders. The 1947 Partition Plan determined citizenship based on residency, such that Arabs and Jews residing in Palestine but not in Jerusalem would obtain citizenship in the state in which they are resident. Professor of Law at Boston University Susan Akram, Omar Barghouti and Ilan Pappé have argued that Palestinian refugees from the envisioned Jewish State were entitled to normal Israeli citizenship based on laws of state succession.

Arab states

See also: Casablanca Protocol

Most Palestinian refugees live either in the West Bank or Gaza Strip, or the three original "host countries" of Jordan, Lebanon and Syria who unwillingly accepted the first wave of refugees in 1948; these refugees are supported by UNRWA. The small number of refugees who settled in Egypt or Iraq were supported directly by those countries' governments. Over the last seven decades, a number of refugees have migrated to other Arab states, particularly the Arab states of the Gulf, primarily as economic migrants.

Arab states' view of Palestinian refugees has varied over time. Arab governments have often supported the refugees in the name of Arab unity, or because they viewed the Palestinians as an important source of skilled human capital to support their economic development. However, Arab governments have also frequently "despised" the Palestinian refugees – either viewing them as a threat to demographic balance (as in Lebanon), or because of the "political message of freedom and emancipation that their ‘Palestinian-ness’ carried", or else because in some countries' history Palestinians have been "somewhat associated with strife and unrest".

Palestinian refugees have taken citizenship in other Arab states, most notably in Jordan. However, the conferring of citizenship is a sensitive topic, as "it is often perceived as allowing Israel to evade its responsibility towards the refugees".

Tashbih Sayyed, a fellow of the Foundation for Defense of Democracies, criticized Arab nations of violating human rights and making the children and grandchildren of Palestinian refugees second class citizens in Lebanon, Syria, or the Gulf States, and said that the UNRWA Palestine refugees "cling to the illusion that defeating the Jews will restore their dignity".

Palestinian views

Most Palestine refugees claim a Palestinian right of return. In lack of an own country, their claim is based on Article 13 of the Universal Declaration of Human Rights (UDHR), which declares that "Everyone has the right to leave any country including his own, and to return to his country", although it has been argued that the term only applies to citizens or nationals of that country. Although all Arab League members at the time (1948) - Egypt, Iraq, Lebanon, Saudi Arabia, Syria, and Yemen - voted against the resolution, they also cite the article 11 of United Nations General Assembly Resolution 194, which "Resolves that the refugees wishing to return to their homes and live at peace with their neighbors should be permitted to do so at the earliest practicable date, and that compensation should be paid for the property of those choosing not to return [...]." However it is currently a matter of dispute whether Resolution 194 referred only to the estimated 50,000 remaining Palestine refugees from the 1948 Palestine War, or additionally to their UNRWA-registered 4,950,000 descendants. The Palestinian National Authority supports this claim, and has been prepared to negotiate its implementation at the various peace talks. Both Fatah and Hamas hold a strong position for a claimed right of return, with Fatah being prepared to give ground on the issue while Hamas is not. However, a report in Lebanon's Daily Star newspaper in which Abdullah Muhammad Ibrahim Abdullah, the Palestinian ambassador to Lebanon and the chairman of the Palestinian Legislative Council's Political and Parliamentary Affairs committees, said the proposed future Palestinian state would not be issuing Palestinian passports to UNRWA Palestine refugees – even refugees living in the West Bank and Gaza.

An independent poll by Khalil Shikaki was conducted in 2003 with 4,500 Palestinian refugee families of Gaza, West Bank, Jordan and Lebanon. It showed that the majority (54%) would accept a financial compensation and a place to live in West Bank or Gaza in place of returning to the exact place in modern-day Israel where they or their ancestors lived (this possibility of settlement is contemplated in the Resolution 194). Only 10% said they would live in Israel if given the option. The other third said they would prefer to live in other countries, or rejected the terms described. However, the poll has been criticized as "methodologically problematic" and "rigged". In 2003, nearly a hundred refugee organizations and NGOs in Lebanon denounced Shikaki's survey, as no local organization was aware of its implementation in Lebanon.

In a 2 January 2005 opinion poll conducted by the Palestinian Association for Human Rights involving Palestinian refugees in Lebanon:

• 96% refused to give up their right of return
• 3% answered contrary
• 1% did not answer

The Oslo Accords

Upon signing the Oslo Accords in 1993, Israel, the EU and the US recognized PLO as the legitimate representative of the Palestinian people. In return, Yasser Arafat recognized the State of Israel and renounced terrorism. At the time, the accords were celebrated as a historic breakthrough. In accordance with these agreements, the Palestinian refugees began to be governed by an autonomous Palestinian Authority, and the parties agreed to negotiate the permanent status of the refugees, as early as 1996. However, events have halted the phasing process and made the likelihood of a future sovereign Palestinian state uncertain. In another development, a rift developed between Fatah in the West-Bank and Hamas in Gaza after Hamas won the 2006 elections. Among other differences, Fatah officially recognizes the Oslo Accords with Israel, whereas Hamas does not.

United States

As of May 2012, the United States Senate Appropriations Committee approved a definition of a Palestine refugee to include only those original Palestine refugees who were actually displaced between June 1946 and May 1948, resulting in an estimated number of 30,000.

Vibronic spectroscopy

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Vibronic_spectroscopy

Vibronic spectroscopy is a branch of molecular spectroscopy concerned with vibronic transitions: the simultaneous changes in electronic and vibrational energy levels of a molecule due to the absorption or emission of a photon of the appropriate energy. In the gas phase, vibronic transitions are accompanied by changes in rotational energy also.

Vibronic spectra of diatomic molecules have been analysed in detail; emission spectra are more complicated than absorption spectra. The intensity of allowed vibronic transitions is governed by the Franck–Condon principle. Vibronic spectroscopy may provide information, such as bond length, on electronic excited states of stable molecules. It has also been applied to the study of unstable molecules such as dicarbon, C₂, in discharges, flames and astronomical objects.

Principles

See also: Rotational-vibrational spectroscopy and Rotational spectroscopy

Electronic transitions are typically observed in the visible and ultraviolet regions, in the wavelength range approximately 200–700 nm (50,000–14,000 cm⁻¹), whereas fundamental vibrations are observed below about 4000 cm⁻¹. When the electronic and vibrational energy changes are so different, vibronic coupling (mixing of electronic and vibrational wave functions) can be neglected and the energy of a vibronic level can be taken as the sum of the electronic and vibrational (and rotational) energies; that is, the Born–Oppenheimer approximation applies. The overall molecular energy depends not only on the electronic state but also on vibrational and rotational quantum numbers, denoted v and J respectively for diatomic molecules. It is conventional to add a double prime (v", J") for levels of the electronic ground state and a single prime (v', J') for electronically excited states.

Each electronic transition may show vibrational coarse structure, and for molecules in the gas phase, rotational fine structure. This is true even when the molecule has a zero dipole moment and therefore has no vibration-rotation infrared spectrum or pure rotational microwave spectrum.

It is necessary to distinguish between absorption and emission spectra. With absorption the molecule starts in the ground electronic state, and usually also in the vibrational ground state ${\displaystyle v^{″}=0}$ because at ordinary temperatures the energy necessary for vibrational excitation is large compared to the average thermal energy. The molecule is excited to another electronic state and to many possible vibrational states ${\displaystyle v^{\prime }=0,1,2,3,...}$ . With emission, the molecule can start in various populated vibrational states, and finishes in the electronic ground state in one of many populated vibrational levels. The emission spectrum is more complicated than the absorption spectrum of the same molecule because there are more changes in vibrational energy level.

The Morse potential (blue) and harmonic oscillator potential (green). The potential at infinite internuclear distance is the dissociation energy for pure vibrational spectra. For vibronic spectra there are two potential curves (see Figure at right), and the dissociation limit is the upper state energy at infinite distance.

 

Energy level diagram illustrating the Franck–Condon principle. Transitions between ${\displaystyle v^{″}=0}$ and ${\displaystyle v^{\prime }=2}$ are favored

For absorption spectra, the vibrational coarse structure for a given electronic transition forms a single progression, or series of transitions with a common level, here the lower level ${\displaystyle v^{″}=0}$. There are no selection rules for vibrational quantum numbers, which are zero in the ground vibrational level of the initial electronic ground state, but can take any integer values in the final electronic excited state. The term values ${\displaystyle G(v)}$ for a harmonic oscillator are given by

${\displaystyle G(v)={\overline{\nu }}_{electronic}+{\omega }_e(v+\frac{1}{2})}$

where v is a vibrational quantum number, ω_e is the harmonic wavenumber. In the next approximation the term values are given by

${\displaystyle G(v)={\overline{\nu }}_{electronic}+{\omega }_e(v+\frac{1}{2})-{\omega }_e{\chi }_e(v+\frac{1}{2}{)}^2}$

where χ_e is an anharmonicity constant. This is, in effect, a better approximation to the Morse potential near the potential minimum. The spacing between adjacent vibrational lines decreases with increasing quantum number because of anharmonicity in the vibration. Eventually the separation decreases to zero when the molecule photo-dissociates into a continuum of states. The second formula is adequate for small values of the vibrational quantum number. For higher values further anharmonicity terms are needed as the molecule approaches the dissociation limit, at the energy corresponding to the upper (final state) potential curve at infinite internuclear distance.

The intensity of allowed vibronic transitions is governed by the Franck–Condon principle. Since electronic transitions are very fast compared with nuclear motions, vibrational levels are favored when they correspond to a minimal change in the nuclear coordinates, that is, when the transition is "vertical" on the energy level diagram. Each line has a finite linewidth, dependent on a variety of factors.

Vibronic spectra of diatomic molecules in the gas phase have been analyzed in detail. Vibrational coarse structure can sometimes be observed in the spectra of molecules in liquid or solid phases and of molecules in solution. Related phenomena including photoelectron spectroscopy, resonance Raman spectroscopy, luminescence, and fluorescence are not discussed in this article, though they also involve vibronic transitions.

Diatomic molecules

The vibronic spectra of diatomic molecules in the gas phase also show rotational fine structure. Each line in a vibrational progression will show P- and R- branches. For some electronic transitions there will also be a Q-branch. The transition energies, expressed in wavenumbers, of the lines for a particular vibronic transition are given, in the rigid rotor approximation, that is, ignoring centrifugal distortion, by

${\displaystyle G(J^{\mathrm{\prime }},J^{\mathrm{\prime }\mathrm{\prime }})={\overline{\nu }}_{v^{\mathrm{\prime }}-v^{\mathrm{\prime }\mathrm{\prime }}}+B^{\mathrm{\prime }}{J}^{\mathrm{\prime }}(J^{\mathrm{\prime }}+1)-B^{\mathrm{\prime }\mathrm{\prime }}{J}^{\mathrm{\prime }\mathrm{\prime }}(J^{\mathrm{\prime }\mathrm{\prime }}+1)}$

Here B are rotational constants and J are rotational quantum numbers. (For B also, a double prime indicates the ground state and a single prime an electronically excited state.) The values of the rotational constants may differ appreciably because the bond length in the electronic excited state may be quite different from the bond length in the ground state, because of the operation of the Franck-Condon principle. The rotational constant is inversely proportional to the square of the bond length. Usually B′ < B′′ as is true when an electron is promoted from a bonding orbital to an antibonding orbital, causing bond lengthening. But this is not always the case; if an electron is promoted from a non-bonding or antibonding orbital to a bonding orbital, there will be bond-shortening and B′ > B′′.

Fortrat diagram created with B′=0.8, B′′=1, showing displacement of rotational lines from the vibrational line position (at 0 cm⁻¹). Centrifugal distortion is ignored in this diagram.

The treatment of rotational fine structure of vibronic transitions is similar to the treatment of rotation-vibration transitions and differs principally in the fact that the ground and excited states correspond to two different electronic states as well as to two different vibrational levels. For the P-branch ${\displaystyle J^{\mathrm{\prime }}=J^{\mathrm{\prime }\mathrm{\prime }}-1}$, so that

${\displaystyle {\overline{\nu }}_P={\overline{\nu }}_{v^{\prime }-v^{″}}+B^{\prime }(J^{″}-1)J^{″}-B^{″}{J}^{″}(J^{″}+1)}$

${\displaystyle ={\overline{\nu }}_{v^{\prime }-v^{″}}-(B^{\prime }+B^{″})J^{″}+(B^{\prime }-B^{″}){J^{″}}^2}$

Similarly for the R-branch ${\displaystyle J^{″}=J^{\prime }-1}$, and

${\displaystyle {\overline{\nu }}_R={\overline{\nu }}_{v^{\prime }-v^{″}}+B^{\prime }{J}^{\prime }(J^{\prime }+1)-B^{″}{J}^{\prime }(J^{\prime }-1)}$

${\displaystyle ={\overline{\nu }}_{v^{\prime }-v^{″}}+(B^{\prime }+B^{″})J^{\prime }+(B^{\prime }-B^{″}){J^{\prime }}^2}$

Thus, the wavenumbers of transitions in both P- and R- branches are given, to a first approximation, by the single formula

${\displaystyle {\overline{\nu }}_{P,R}={\overline{\nu }}_{v^{\mathrm{\prime }},v^{\mathrm{\prime }\mathrm{\prime }}}+(B^{\mathrm{\prime }}+B^{\mathrm{\prime }\mathrm{\prime }})m+(B^{\mathrm{\prime }}-B^{\mathrm{\prime }\mathrm{\prime }})m^2,m=\pm 1,\pm 2etc.}$

Here positive m values refer to the R-branch (with m = +J ′ = J'' +1) and negative values refer to the P-branch (with m = -J ′′). The wavenumbers of the lines in the P-branch, on the low wavenumber side of the band origin at ${\displaystyle {\overline{\nu }}_{v^{\mathrm{\prime }},v^{\mathrm{\prime }\mathrm{\prime }}}}$, increase with m. In the R-branch, for the usual case that B′ < B′′, as J increases the wavenumbers at first lie increasingly on the high wavenumber side of the band origin but then start to decrease, eventually lying on the low wavenumber side. The Fortrat diagram illustrates this effect. In the rigid rotor approximation the line wavenumbers lie on a parabola which has a maximum at

${\displaystyle x=-\frac{B^{\mathrm{\prime }}+B^{\mathrm{\prime }\mathrm{\prime }}}{2(B^{\mathrm{\prime }}-B^{\mathrm{\prime }\mathrm{\prime }})}}$

The line of highest wavenumber in the R-branch is known as the band head. It occurs at the value of m which is equal to the integer part of x, or of (x+1).

When a Q- branch is allowed for a particular electronic transition, the lines of the Q-branch correspond to the case ∆J=0, J′=J′′ and wavenumbers are given by

${\displaystyle {\overline{\nu }}_Q={\overline{\nu }}_{v^{\prime },v^{″}}+(B^{\prime }-B^{″})J(J+1)J=1,2etc.}$

The Q-branch then consists of a series of lines with increasing separation between adjacent lines as J increases. When B′<B′′ the Q-branch lies to lower wavenumbers relative to the vibrational line.

Predissociation

The phenomenon of predissociation occurs when an electronic transition results in dissociation of the molecule at an excitation energy less than the normal dissociation limit of the upper state. This can occur when the potential energy curve of the upper state crosses the curve for a repulsive state, so that the two states have equal energy at some internuclear distance. This allows the possibility of a radiationless transition to the repulsive state whose energy levels form a continuum, so that there is blurring of the particular vibrational band in the vibrational progression.

Applications

Spectrum of the blue flame from a butane torch showing excited molecular radical band emission and Swan bands due to C₂.

 

See also: List of interstellar and circumstellar molecules

The analysis of vibronic spectra of diatomic molecules provides information concerning both the ground electronic state and the excited electronic state. Data for the ground state can also be obtained by vibrational or pure rotational spectroscopy, but data for the excited state can only be obtained from the analysis of vibronic spectra. For example, the bond length in the excited state may be derived from the value of the rotational constant B′. In addition to stable diatomic molecules, vibronic spectroscopy has been used to study unstable species, including CH, NH, hydroxyl radical, OH, and cyano radical, CN. The Swan bands in hydrocarbon flame spectra are a progression in the C-C stretching vibration of the dicarbon radical, C₂ for the ${\displaystyle d^3{\mathrm{\Pi }}_u\iff a^3{\mathrm{\Pi }}_g}$ electronic transition. Vibronic bands for 9 other electronic transitions of C₂ have been observed in the infrared and ultraviolet regions.

Polyatomic molecules and ions

Formaldehyde

For polyatomic molecules, progressions are most often observed when the change in bond lengths upon electronic excitation coincides with the change due to a ″totally symmetric″ vibration. This is the same process that occurs in resonance Raman spectroscopy. For example, in formaldehyde (methanal), H₂CO, the n → π* transition involves excitation of an electron from a non-bonding orbital to an antibonding pi orbital which weakens and lengthens the C-O bond. This produces a long progression in the C-O stretching vibration. Another example is furnished by benzene, C₆H₆. In both gas and liquid phase the band around 250 nm shows a progression in the symmetric ring-breathing vibration.

Absorption spectrum of an aqueous solution of potassium permanganate

As an example from inorganic chemistry the permanganate ion, MnO⁻
₄, in aqueous solution has an intense purple colour due to an O → Mn ligand-to-metal charge transfer band (LMCT) in much of the visible region. This band shows a progression in the symmetric Mn-O stretching vibration. The individual lines overlap each other extensively, giving rise to a broad overall profile with some coarse structure.

Progressions in vibrations which are not totally symmetric may also be observed.

d-d electronic transitions in atoms in a centrosymmetric environment are electric-dipole forbidden by the Laporte rule. This will apply to octahedral coordination compounds of the transition metals. The spectra of many of these complexes have some vibronic character. The same rule also applies to f-f transitions in centrosymmetric complexes of lanthanides and actinides. In the case of the octahedral actinide chloro-complex of uranium(IV), UCl₆²⁻ the observed electronic spectrum is entirely vibronic. At the temperature of liquid helium, 4K, the vibronic structure was completely resolved, with zero intensity for the purely electronic transition, and three side-lines corresponding to the asymmetric U-Cl stretching vibration and two asymmetric Cl-U-Cl bending modes. Later studies on the same anion were also able to account for vibronic transitions involving low-frequency lattice vibrations.