relaxation and vibrational fluorescence of wo in solid neon

17 July 1998

Ž .Chemical Physics Letters 291 1998 291–299

Relaxation and vibrational fluorescence of WO in solid neon

Martin Lorenz, Jurgen Agreiter, Nicola Caspary, Vladimir E. Bondybey )¨Institut fur Physikalische und Theoretische Chemie, Technische UniÕersitat Munchen, Lichtenbergstraße 4, D-85748 Garching, Germany¨ ¨ ¨

Received 28 April 1998

Abstract

Studies of laser-induced fluorescence of tungsten oxides in solid neon at 6 K provide interesting insights into themechanisms of WO relaxation. The relaxation occurs by interelectronic cascade between the vibrational manifolds of excitedelectronic states, eventually reaching the Õ

Ys7 of the ground electronic state. Remarkably, for the heavy WO oxide, thenon-radiative processes are surprisingly inefficient, with relaxation in the ground state apparently occurring predominantlyby sequential vibrational fluorescence. The relaxation pathways of WO are discussed and compared with relaxation inlighter, first-row molecules exemplified by CN. q 1998 Elsevier Science B.V. All rights reserved.

1. Introduction

Low-temperature rare-gas solids are a usefulmedium for studies of molecular relaxation and

w xnon-radiative processes 1–4 . While relaxation inthe condensed phase is usually fast, often proceedingon a picosecond or even subpicosecond timescale, atlow temperatures in solid rare gases it often slowsdown remarkably. In small molecules often vibra-tionally unrelaxed ‘hot’ electronic fluorescence isobserved and in particular in the ground states oflight molecules the non-radiative relaxation may beslow. In a number of light molecules, mostly involv-ing hydrides or simple compounds of first-row ele-ments, vibrational fluorescence was also observed

w xand investigated 5–10 . The diatomic CN is anexample of a molecule which exhibits a slow relax-

) Corresponding author.E-mail: [email protected]

ation and for which intense vibrational fluorescencew xis observed 11 .

Relaxation for molecules involving heavier ele-ments is usually efficient, and even in electronicspectra one mostly sees only vibrationally relaxedfluorescence or phosphorescence. Particularly intransition metals, with their dense manifold of elec-tronic states, fast relaxation is usually the rule andonly in relatively few cases extensive ‘hot’ fluores-cence is detected. We have recently examined thespectroscopy and photophysics of the oxides of the

w xthird-row transition metal tungsten 12 . Rather sur-prisingly we have observed that, in spite of theessential differences between the two molecules, theenergy relaxation in WO parallels the processes pre-

w xviously detected in CN 13,14 . Extensive vibra-tionally unrelaxed emission from several electronicstates is observed and even more surprisingly, quiteintense ground-state vibrational fluorescence is eas-ily detectable. Our observations of the WO relax-ation behavior are the subject of this Letter.

0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.Ž .PII: S0009-2614 98 00605-8

( )M. Lorenz et al.rChemical Physics Letters 291 1998 291–299292

2. Experimental

Tungsten and its oxides are high-melting refrac-tory materials and their vaporization requires temper-atures ranging up to 3000 K. Here we employ atechnique developed in our laboratory more than a

w xdecade ago 15 ; laser vaporization of the metal inthe presence of a large excess of cold carrier gas,neon gas being used to grow the matrix. The fre-

Ž .quency-doubled pulses of an Nd-YAG laser 30 mJused to vaporize a tungsten target are synchronizedwith the opening of a fast valve admitting pulses ofthe carrier gas. The valve, with opening times of 0.1ms, is operated with a backing pressure of 5 bar of a

Ž 18 .500:1 mixture of neon with O or O . The neon2 2

gas with the products and oxides formed in thevaporization fixture are then condensed on a 6 Ksilver-coated copper substrate.

The deposits are examined spectroscopically withthe help of a Bruker IFS 120 HR FT instrument witha spectral range from 500 to 30000 cmy1. Spectrahave been recorded with resolutions of 0.5 cmy1 and

y1 Ž .0.06 cm IR . To excite the sample fluorescence,either an argon ion laser pumped dye laser, or aTi-Sapphire laser were used. A PMT detector wasused in the visible and UV range, while germaniumand indium antimonide detectors were available forstudies in the near infrared range.

3. Results and discussion

3.1. WO electronic structure

Tungsten oxides were studied in solid rare gasesmore than 30 years ago by Weltner and McLeodw x16 , who observed a number of WO electronic statesin the visible region, and some of these states weresome 10 years later also investigated by Samoilova

w xet al. in the gas phase 17 . Nelin and Bauschlicherhave more recently investigated WO by ab initiotechniques, which suggested that the ground state isformally a 5

P state and predicted the excited 5S and

7S states in the visible region. Their calculations

yielded another, low lying 7P state around 7400

y1 w xcm in the near infrared 18 . The electronic struc-ture of transition metals and the ability of ab initiomethods to predict it is a topic of considerable

current interest and we have therefore decided tore-examine the WO spectroscopy using absorptionand fluorescence techniques, in order to gain addi-tional spectral information and characterize possibletransitions in the infrared.

The solid neon matrices, generated by laser vapor-izing tungsten in the presence of a small amount ofoxygen, contained diatomic tungsten oxide, as evi-denced by infrared absorptions. Not only the oxygen,but also the tungsten isotopes are clearly resolved.Thus absorptions due to 184 W16O and 184 W18O oc-cur at 1056.49 and 1001.25 cmy1, respectively. Inagreement with Weltner and McLeod, we observe inthe visible the states he denotes A–F, as well asseveral new states. In addition, in the near infraredwe have detected a group of states. Several progres-sions occur with origins between 7403.8 and 8007.6cmy1, with by far the strongest n absorption oc-00

y1 Ž y1 18 .curring at 7527.76 cm 7529.08 cm for W O .While some of the weaker bands appearing here aredue to WO in ‘perturbed’ matrix sites, there areclearly at least three absorbing states present in thisregion. They may be components of the 7

P statepredicted by Nelin and Bauschlicher. We will report

Table 1Molecular constants of 184 W16O in solid neon

aState T v v xe e e ey1 y1 y1Ž . Ž . Ž .cm cm cm

5 bX P 0.0 1064.49 3.9987 cŽ .a P?, red site 7454.3 957.9 3.277 cŽ .a P? 7578.6 959.6 3.767 cŽ .b P? 7761.1 958.7 3.557 cŽ .c P? 8062.1 956.1 3.26

cd 10938.7 1022.3ce 12075.4 970.5cf 14132.7 1084.3

5 7Ž .A S or S? 14116.9 1051.2 10.84B 17336.9 929.5 y1.07C 19267.5 932.6 2.76Ž .D red site 20799.8 996.8 8.22

D 20821.4 998.7 8.51E 21538.2 971.3 8.49F 23439.4 955.4 9.52G 23852.7 934.2 y5.1

X c,dH 27037.3 931.6 13.15

a w xSymmetries as proposed by Nelin and Bauschlicher 18 .bv y sy0.0034 cmy1 and for the X5

P ground state.e ec Previously not observed.d Ž y1 .Not identical with the H-state T s30446 cm of Samoilova00

w xet al. 17 .

( )M. Lorenz et al.rChemical Physics Letters 291 1998 291–299 293

Fig. 1. W16O potential curves and vibrational levels in the rangeof 0–22000 cmy1. The r s are obtained by Franck–Condone

w xanalysis or are taken, where available, from Samoilova et al. 17 .

the details of our spectroscopic observations else-w xwhere 12 and the present Letter will focus mainly

on the WO electronic and vibrational relaxation. Wesummarize the information about the observed elec-tronic states and their molecular constants necessaryfor discussing the relaxation processes in Table 1and show the energy level structure of WO schemati-cally in Fig. 1.

3.2. Fluorescence and relaxation of the excited WOelectronic states in the Õisible

Fluorescence spectra can often provide a valuablecomplementary contribution to absorption studies.As explained in the experimental section, we haveused our tunable laser sources to excite the neonmatrix absorptions due to WO. Such an excitation ata variety of frequencies ranging from about 10000 to25000 cmy1 resulted in a fairly strong visible andnear infrared fluorescence, whose intensity and spec-tral distribution depended sensitively upon the exci-tation wavelength and upon the isotopic species ex-cited. While some levels were emitting efficiently,

others were conspicuously absent, indicative of acomplex isotopically dependent pattern of relaxation.This relaxation process is quite complex and in-volves vibrational manifolds of the numerous elec-tronic states present in this region, both those ob-served in absorption, and probably of other, ‘dark’states. Rates of such interelectronic relaxation pro-cesses are known to depend sensitively upon thequantum mechanical coupling between the differentelectronic states, but also on the ‘energy gap’ be-tween the relaxing levels and are often governed bythe presence of accidental resonances or near reso-nances.

When the lower energy levels at the red end ofthe spectrum reported by Weltner and McLeod areexcited, the fluorescence is relatively simple andeasy to understand. Thus exciting the sample in theÕs0 level of the W18 O in the B-state of Weltnerand McLeod at 17273.9 cmy1, one observes emis-sion from the excited level into Õ

Y s0, 1 and 2levels of the ground electronic state. A further pro-gression originating from a level at 16980.4 cmy1

into the ÕY s1–4 ground-state levels can also easily

be identified as being due to the ÕX s3 A-state level,

293.5 cmy1 lower in energy. The 16980.44 cmy1 isalso seen weakly in absorption, although not previ-ously observed by Weltner and McLeod. Excitationof the corresponding Õ

X s0 B-state level of W16OŽ y1 .17280.5 cm , on the other hand, produces almostno B-state emission, but mainly weak emissionsfrom the Õ

X s3 and 2 A-state levels. Owing to theisotopic shift, in the W16O the Õ

X s3 A-level is only150 cmy1 below the Õ

X s0 B-level and the smallerenergy gap results in a more efficient depopulationof the B-state.

As one proceeds towards higher energies, thefluorescence gets progressively more complex. For

X Ž y1 .instance, excitation of the Õ s2 D 22635.9 cm ,produces not only a weak resonant emission from theexcited level, but extensive relaxed fluorescence froma number of other levels. No emission is seen fromÕ

Y s1 D, but several bands originating from ÕX s0

Ž y1 .E 21493.0 cm and a fairly strong progressionX Ž y1 .from Õ s0 D 20789.3 cm and a weaker oneX Ž y1 .from Õ s0 B 17274.1 cm are seen. Another

rather long progression originates from the A ÕX s6

Ž y1 .level 19675.5 cm and extends down to 11867.9cmy1 into the Õ

Y s8 ground state. In addition to the


clearly identifiable progressions, numerous veryweak, sharp bands are observed confirming that inaddition to states previously detected by Weltner andMcLeod, several other ‘dark’ or very weakly absorb-ing states must be present in this region and partici-pate in the relaxation process.

It may be noted that some weak bands appearingin the absorption spectrum are due to ‘site effects’.The bands due to ‘sites’ are, however, easily identi-fied, since unlike excitation of WO in the ‘main’site, their excitation results in spectrally shifted emis-sion bands. Thus all the bands of the D progressionexhibit a much weaker satellite some 24 cmy1 lowerin energy, whose excitation produces fluorescencebands uniformly shifted by the same 24 cmy1.

3.3. Infrared phosphorescence

When the visible bands are excited, one observesnot only visible fluorescence, but also a strong emis-sion in the near infrared range, clearly due to theband system also observed in absorption. Representa-tive spectra obtained with the Ge detector by tuningthe exciting laser to the B–X 0–0 transition at 17281and 17274 cmy1 for W16O and W18 O, respectively,are shown in Fig. 2. Unlike the visible fluorescence,

the emission appearance is essentially independent ofwhich of the vibronic bands of the visible or near IRprogressions is excited by the laser and from themultiple ‘systems’ detected in the absorption spec-trum, only the strongest one with its origin at 7527.6cmy1 appears. As already noted above, it may bedue to the ‘spin forbidden’ phosphorescence fromthe 7

P state, although L is probably not well de-fined in a molecule as heavy as WO. In contrast withthe visible fluorescence, the near infrared emission isrelatively simple and easy to analyse to yield themolecular constants of the ‘a’-state. Interestingly, theinfrared spectrum is at least one order of magnitudemore intense in the heavier W18 O isotopic molecule,which appears with appreciable intensity even in thenatural isotopic abundance samples.

The spectra show that vibrational relaxation ofWO in neon is relatively slow even in the electroni-cally excited ‘a’-state, since not only Õ

X s0, but alsoemission originating from vibrationally unrelaxedlevels is observed. In the case of the stronger W18 Ofluorescence, levels up to Õ

X s3 are detected inemission and the spectra can be followed with anInSb detector down to about 3300 cmy1 in theinfrared, with the progressions in the ground-state

Fig. 2. NIR absorption and phosphorescence of tungsten oxide. The upper trace shows progressions of several NIR states, with the strongestŽ7 . 5 y1absorption, denoted a P? §X P, occurring at T s7527.8 cm . The lower two traces display the phosphorescence originating from00

Ž7 .the ‘a’ P? state of both tungsten oxides, obtained after excitation of the B Õs0 state.


vibrational frequency extending to ÕY s5. The relax-

ation of the lighter W16O is apparently more efficientand the corresponding emission appreciably weaker.In this case, besides the vibrationless level onlyÕ

X s1 emission is observed.

3.4. Vibrational fluorescence of WO

At the red end of the spectrum obtained with theindium antimonide detector one can observe, startingat around 3000 cmy1, a slowly rising background,which reaches a maximum near 1900 cmy1 and thendrops to zero due to a cutoff of the InSb detectorŽ .see Fig. 3 . This broad maximum is clearly due tothe high energy end of the ‘black-body’ radiationfrom the room temperature apparatus walls. Super-imposed over this maximum one can see in theexperiment employing natural abundance oxygenthree strong absorptions at 2312.4, 2330.6 and 2347.4cmy1, as well as a group of eight moderately strongemission bands at 1991.5, 2007.8, 2024.1, 2040.4,2056.6, 2072.7, 2088.8 and 2104.9 cmy1. The threeabsorption bands are clearly due to three isotopicspecies of carbon dioxide, C18O , 16OC18O and2

C16O probably formed due to the oxygen reaction2

with a minor carbon impurity in the tungsten target.

The emission bands are readily identified as vibra-tional overtone fluorescence of diatomic WO, withthe eight strong bands being the 9–7, 8–6, 7–5, 6–4,5–3, 4–2, 3–1 and 2–0 transitions and in additionalso the very weak 10–8 bands are detected. Thisassignment is quite unambiguous and is further con-firmed by the clearly resolved tungsten isotopicstructure. In an experiment with 18-oxygen, the ap-propriately shifted 7–5, 6–4, . . . , 2–0 bands areobserved. A careful examination of the region around3000 cmy1 revealed also the DÕs3 sequence. Fi-nally, as one can see in Fig. 4, the DÕs1 transitionscould easily be observed between 1000 and 1100cmy1 using a mercury–cadmium telluride detector.

As noted above, the visible bands of WO exhibit,in agreement with Weltner and McLeod, strong in-terstate interactions and perturbations, so that onlythe W18 O spectrum can be reasonably fitted to obtainthe spectroscopic constants of the excited states.However, employing the appropriate isotopic rela-tionships, the ground-state vibrational data includingsome 80 transitions for eight different isotopic speciesof WO and involving levels up to Õ

Y s9, can bealmost perfectly fitted. A Morse potential with v se

y1 y1 Ž1064.65 cm and v x s4.046 cm for thee e184 16 .W O isotopomer reproduces the observed levels

Fig. 3. Vibrational overtone fluorescence of W16O. The main trace displays the DÕs2 and the weaker DÕs3 transitions of ground stateW16O in solid neon. Superposed by black-body radiation, the 9–7, 8–6, . . . , 2–0 bands are easily observed. The 8–5, 7–4, 6–3 and 5–2transitions appear near 3000 cmy1 , with about one order of magnitude weaker intensity. The insert shows the strong 8–6 transition withtungsten isotopical resolution. Peaks denoted with an asterisk are probably due to impurities.


Fig. 4. Vibrational fluorescence of tungsten oxide in neon. These two graphs show the DÕs1 ground-state emission of W16O and W18 Oafter excitation at B Õs0. The spectra, measured with a mercury cadmium telluride detector, are background corrected by subtracting thedominating black-body radiation.

with a RMS deviation of -0.09 cmy1. While aŽ y1 .somewhat better fit RMS-0.03 cm is obtained

y1 Žwith v y sy0.0034 cm the other constantse e

change slightly to v s1064.49 cmy1 and v x se e ey1 .3.998 cm , the inclusion of v z or higher termse e

results in no further improvement.

3.5. X-state Õibrational relaxation of WO

The observation of ground-state vibrational fluo-rescence of WO is rather surprising for at least tworeasons. In the first place, while one occasionallyobserves vibrationally unrelaxed fluorescence inelectronic transitions of similar matrix isolated smallmetal containing molecules, such observations in theinfrared are quite uncommon. In view of the muchlower oscillator strengths and longer radiative life-times associated with vibrational transitions, vibra-tional fluorescence is usually observed only for hy-drides and light molecules involving first row atoms,with high vibrational frequencies usually above1500–2000 cmy1. In the second place, the ground-state potential of WO is relatively harmonic andtherefore one might not expect strong overtone tran-sitions.

Unfortunately, we are not equipped for the direct

measurement of the vibrational lifetimes. Estimatingabsolute emission quantum yields is always difficult,but semiquantitative consideration of the observedband intensities suggests that it is high and that forevery excited molecule several IR photons may beemitted. Such a high quantum yield of vibrationalemission indicates that the non-radiative relaxationof the WO ground-state vibrational levels must beexceedingly slow. The fact that life times are long inthe WO case is, however, also demonstrated by thefact that absorptions originating in excited vibra-tional levels can easily be detected. An inefficientenergy transfer between the light neon atoms and themuch heavier WO molecule might be a contributingfactor. In many molecules where strong vibrationalfluorescence is observed, e.g. OH, NH, CH F or3

hydrogen halides, it has been demonstrated that therelaxation involves a V–R process, with the rota-tional modes of the guest molecule being the primary

w xacceptors of the vibrational energy 19 . The rota-Žtional constants of WO are much smaller B f0.415e

y1 .cm and the rotations probably contribute little,since high rotational levels would have to be in-volved.

The appearance of the overtones in the vibrationalspectrum will, of course, be aided by the n 4 depen-dence of the coefficient of spontaneous emission,


which will favor the DÕs2 and 3 transitions byfactors of 16 and 81, respectively, and thus counter-act the harmonic oscillator selection rules. Eventhough the mechanical anharmonicity of WO is small,the overtones can be favored by electric anharmonic-ity. The WO molecule may be fairly ionic, but atlarger distances it dissociates into neutral atoms andits dipole is surely not a linear function of r.

The observed relaxation processes, where absorp-tion of a single visible photon is followed by emis-sion of a series of IR photons seems thus far to beunique. It may be noted that similar processes, that isUV pumping, followed by an internal conversion andground-state vibrational fluorescence are thought tobe responsible for the so-called unidentified infrared

Ž .emission bands UIE , with polycyclic aromatic hy-Ž .drocarbons PAH being the suggested carriers

w x20,21 . The search for emission of these species in aneon matrix might be of interest. The excellentsignal to noise ratio and clearly high quantum yieldsof infrared fluorescence observed in WO raise thequestion if such a behavior might not be much morewidespread, and if studies of vibrational emissioncould not provide a useful general tool for charac-terizing small metal oxides, carbides or nitrides.

3.6. Pathways of relaxation of WO in solid neon

One of the molecules for which strong infraredemission has been detected, and whose relaxation inmatrices has been most extensively investigated isprobably the CN radical. Even though the propertiesand electronic structures of the two molecules couldhardly be much more different and while WO has avibrational frequency about half of that of CN, therelaxation pathways of the two molecules in thematrix are remarkably similar. In the higher energyregion, the relaxation proceeds in both molecules byan interelectronic process involving the vibrationalmanifolds of several excited electronic states. Thusas in CN, the excitation zig-zags between the B2

S2 w xand A P electronic states 14 , also WO cris-crosses

in the visible between the several electronic statesavailable at that energy. The non-radiative interelec-tronic transitions are dominated by accidental leveldegeneracies and the rates are governed by Franck–Condon factors and the energy gap law.

When the lowest excited electronic state, the A2P

state in the CN or the ‘a’ WO state is reached, theprocess slows down further and in both moleculesextensive vibrationally unrelaxed, ‘hot’ emission, is

w xobserved 13 . In CN it was shown that rather thanproceeding directly down the vibrational manifold ofthe A-state, further relaxation occurs exclusively byrepeated crossings between the vibrational levels ofthe A2

P state and the nearest X2S ground-state

levels. Similarly it is almost certain that also in theŽ7 .lowest a P? excited state of WO, the relaxation

proceeds by an interelectronic mechanism. In thelight CN molecule, the A2

P spin–orbit constant isŽ y1 .relatively small Asy52.64 cm , with relax-

ation between the components being extremely fastand the splitting has little effect upon the relaxationin the matrix. While in WO the spin–orbit effects aresurely much larger and the components of the widelyseparated ‘septet’ state could be involved in therelaxation, it appears likely that here ground-statelevels are involved, too.

After the relaxation reaches the vibrationless ÕX s

0 level of the lowest electronically excited state, onlyone intramolecular non-radiative process is possible:crossing into one of the nearby ground-state levels.From that point on, the interelectronic channel isclosed and the molecule can only relax intrastate,either non-radiatively, or by emitting infrared pho-tons. Thus in the case of CN, the Õ

Y s4 X2S

ground-state level is populated from ÕX s0 A2

P.Further relaxation proceeds by sequential infraredfluorescence and the ground-state Õ

Y s1–4 levelsappear strongly in emission. In W18 O the vibration-

X Ž7 . y1less Õ s0 a P? level occurs 7529.39 cm abovethe X Õ

Y s0 level, with the nearest ground-statevibrational level being at 6857.7 cmy1, 671.69 cmy1

lower in energy. Quite analogous to the CN relax-ation, as discussed above, the highest level fromwhich vibrational infrared fluorescence is observedis Õ

Y s7. Clearly also here further relaxation appar-ently occurs by sequential vibrational fluorescence.

As noted above, in contrast with the heavierisotope, in W16O the a-state emission is weaker andconversely, the vibrational emission considerablymore efficient. An explanation is easily found byexamining the vibrational level structures of the twooxygen isotopic species, which also provides further

Ž7 .support for the relaxation in the lowest a P? stateproceeding, as in CN, via the ground-state vibra-


tional levels. While in W18O, the ÕX s0–3 are lo-

cated 671, 623, 576 and 526 cmy1 above the closestground-state level, for W16O the numbers are 300,

y1 Ž .252, 205 and 158 cm see Fig. 1 . The smallerenergy gaps in W16O result in a much more efficientrelaxation into the ground-state vibrational manifold,while conversely the larger gaps in W18O lead toenhanced phosphorescence, bypassing the excitedground-state levels.

The involvement of the ground state in a-staterelaxation is then directly evidenced by the observa-

X Ž7 .tion of emission from levels above Õ s0 a P? .As noted above, also Õ

X s8, 9 and very weakly alsoÕ

X s10 X-levels, located 779, 740 and 701 cmy1

above the ÕX s2, 1 and 0 a-state levels, are seen in

emission in W16O. These relatively large ‘energygaps’, combined with poor Franck–Condon factors,make the reverse transition into the a-state once thecrossing into the X-state has occurred inefficient andvibrational emission becomes competitive. On theother hand, in the heavier isotope the correspondinggaps are only 360, 320 and 280 cmy1, respectively.This makes the reverse crossing into the a-state moreefficient compared with direct sequential radiativevibrational relaxation within the ground state.

3.7. Intrastate Õs. interstate Õibrational relaxation

As described above, the relaxation in the quitedissimilar molecules CN and WO proceeds in bothcases via the same interelectronic mechanism, andsimilar observations have been made for a variety ofother systems. In fact, even when the energy gaps forthe two processes are comparable, the interstate pro-cess is in general much more efficient. As explainedpreviously, the physical reasons are to be found inFranck–Condon arguments applied to the localphonon modes and to the response of the lattice tothe change of state of the guest. The nearby latticeatoms experience little change when the guestmolecule changes its vibrational state. This is evi-denced by the vibrational spectra of the guestmolecules, which mostly exhibit only strong ‘zerophonon lines’ and little evidence of phonon side-bands.

On the other hand, the electron density distribu-tion in the neighborhood of the guest molecule

changes much more drastically when an electronictransition takes place, for instance from a S- to aP-state. This again is usually evidenced by the muchmore prominent phonon sidebands associated withelectronic transitions. Thus in the optical A2

Pl

X2S transition in matrices, rather strong phonon

sidebands are observable. In the case of WO, thephonon wings are weaker, but clearly observable.These same Franck Condon factors which channelenergy into the lattice phonons during an opticaltransition between the two electronic states also fa-cilitate the corresponding non-radiative relaxationprocess.

4. Conclusion

Pathways of relaxation of electronically excitedWO molecules in solid neon at 6 K were investi-gated. In spite of the much lower vibrational fre-quency and entirely dissimilar molecular propertiesand structure, the pattern of relaxation processesresembles those found in the much lighter CN radi-cal. The relaxation occurs by interelectronic cascadebetween the vibrational manifolds of excited elec-tronic states, reaching eventually the Õ

Y s7 of theground electronic state. The non-radiative processesin the ground state are surprisingly inefficient, withrelaxation occurring predominantly by sequential vi-brational fluorescence.

Acknowledgements

Financial support of this research by the DeutscheForschungsgemeinschaft and the Fonds der Chemis-chen Industrie is gratefully acknowledged. The au-

Žthors are also indebted to NATO Grant No. CRG.960009 for additional support.

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