731
J. Phys. Chem. 1902, 86, 731-736
Flash Spectroscopy of Multiphoton Excited CF31 W. Fuss Max-Planck-Instnut fOf Ouantenoptlk, D-8046 Garchlng, Munchen, Federal Republic of Germany (Received: August 3, 198 1: I n Fhal Form: October 5, 1981)
CF31 was excited in its absorption bands ul, 2u5, and u2 + u3 by a pulsed C02laser. The vibrational structure thus produced in the UV absorption between 174 and 180 nm ("hot bands") was photographed and assigned. In this way the states populated by the COzlaser were identified. Although the observation did not completely avoid collisons, conclusions about the collisionless process have been possible. In the first two or three infrared absorption steps (depending on the pump frequency) the population was found only in vibrational states which are well in resonance and which are connected by intense infrared transitions. After these absorptions, direct multiphoton transitions seem to follow.
Introduction The basic question of collisionless infrared multiphoton excitation is as follows: How can a polyatomic molecule absorb tens of photons without being driven completely out of resonance owing to anharmonicity? There is agreement that an important contribution is due to the availability of more than one vibrational mode. I t is controversial, however, how this contribution works. A critical review of suggested mechanisms is compiled in ref 1. In ref 2-4 experimental evidence is reported for the participation of modes which are nonresonant in the first step. In levels close to the dissociation limit, even an equipartition of energy over all vibrations is probable. Equipartition has also been postulated for low levels, although this would sometimes be hard to understand in terms of spectroscopy.' In the infrared e m i ~ s i o n ~ and , ~ anti-Stokes Raman4 studies, statements could only be made about the energy or number of quanta in the investigated modes, whereas the exact levels or values of the remaining quantum numbers could only be surmised. For example, the u2 Raman emission of multiphoton excited CF31was observed4J3at -680 cm-l, Le., shifted by 60 cm-' from the fundamental frequency 743.3 ~ m - l .On ~ the basis of the anharmonic constants of ref 5, such a shift would be expected for emission e.g., from states like u2 + 43u3 (energy -10500 cm-') or v2 + 9 u4 ( 10 900 cm-') or from states v2 u3v3 v4u4 of similar energy. Most other states would require higher excitation, e.g., molecules with equipartitional energy distribution would have an anharmoinic shift of 60 cm-' only if excited to -18500 cm-', which is about the dissociation limit. Additional uncertainties of assignment arise because such extrapolations to high energies, using constants derived from low levels, are estimates at most. The difficulty in infrared and Raman vibrational spectroscopy is that spectral shifts (by anharmonicity) are only 1-10 cm-' per unit change of a quantum number, so that many transitions can overlap. In contrast, vibrational structures of electronic transitions are typically shifted by 100 cm-' when a vibrational quantum number changes in the initial electronic state, if bands of given Av ("sequence bands") are compared. Therefore the analysis
+
-
+
-
(1)W.Fuss and K. L. Kompa, Laboratory Report PLF 30,1980;B o g . Quantum Electron., submitted for publication. (2) W. E. Baxch, H. R. Fetterman, and H. R. Schlossberg, Opt. Commum, 15, 358 (1975). (3)W.Fuss, Chem. Phys. Lett., 71,77 (1980). (4)V. N.Bagratashvili, Yu. G. Vainer, V. S. Dolzhikov, S.F. Kol'yakov, A. A. Makarov, L. P. Malyavkin, E. A. Ryabov, E. G. Sil'kis, and V. D. Titov, JETP Lett., 30, 471 (1979);Appl. Phys., 22, 101 (1980).
of new vibrational structure, appearing in visible or UV transitions when molecules are excited by infrared radiation, should provide more details about populated vibrational states. This work concerns an investigation of the 174 nm XIAl C'E transition of CF31, pumped by various COz laser frequencies with energies between and 1 J/cm2. In contrast to the weak and continuous X A absorption at 267 nm studied in ref 6 and 7, the far-UV transition is very intense (for the 0-0 band u = 5 X cm-2 at room temperature) and well structured. The structure is simple, consisting of very short progressions (2 to 3 members) of the three symmetric vibrations. Each populated initial state therefore has a simple spectrum. Hot bands were reduced by cooling to -90 "C. CF31is an excellent candidate for IR preexcited UV flash spectroscopy not only owing to its simple UV spectrum, but also because of the following reasons: (1)the molecule can easily be excited to dissociation by a pulsed COz laser; (2) its vibrational spectroscopy has been extensively studied in the infrared and most level positions are known with sufficient ac~uracy;~ (3) the molecule is small so that the density of states is not very high; (4) one of the excited modes, u2 + u3, is nondegenerate. However, the region around (the nondegenerate) u1 which was also investigated turned out to be complicated due to the closeness of 2 4 and 2 4 .
-
-
Experimental Section The measurement scheme is shown in Figure 1. U1traviolet radiation was produced by an argon-filled flash lamp. The absorption spectra transmitted by the CF31 were photographed by use of a 50-cm vacuum spectrograph equipped with a 2400 groove/" grating. The entrance slit width was, in most experiments, 50 pm corresponding to a spectral slit width of 40 pm or 15 cm-'. The rotational contours are slightly broader. This slit width made five flashes necessary for a good exposure. To recognize the positions of bands, magnified positive copies on highcontrast paper proved to be more helpful than densitometer traces of the negatives. The flash lamp emitted UV radiation at its front end thru a far-UV quartz window. An expansion volume was installed between the windows and the discharge part. The window was flushed by flowing fresh argon slowly thru the flash tube. In this way deposition of absorbing and scattering dust on the windows was reduced. Argon was used although Xe was twice as bright, because the latter (5)W.Fuss, J. Mol. Spectrosc., submitted for publication.
0022-365418212086-073 1$01.25/0 0 1982 American Chemical Society
732 The Journal of Physical Chemisrry, Vol. 86, No. 5, 1982
Fuss
IR 4=+ C Y 1
0c y 1
Ge
F
Figure 1. Measurement setup: (M) vacuum monochromator with photographic film (Ph); the infrared radiation (IR) Is matched and focussed by the two cylindrical lenses (Cyl) through the two germanium windows (Ge) to the cell (C), whose filling line is not shown; It is cooled by a ayocooler (Cryo); the temperature is measured by a thermocouple (PI)(F) ; bandpass filter for transmission of 172 f 20 nm; (El) electrodes for discharge through the flash lamp (FL).
h U
-
VIS
c-_
I
0
I
I
I
,
I
5
,
,
/
,
I
10
us
Figure 2. Temporal shapes and timing of the COPlaser pulse (IR). The flash lamp emission at 174 nm (UV) was measured by a photomultiplier replacing the photographic film in Figure 1; its emission in the visible and near-ultravblet (VIS) was measured by a silicon PIN diode close to the flash lamp, and the discharge current (CUR). For comparison, broken lines show emission and current for 0.9 bar of Ar in the flash lamp, instead of the 1.8-1.9 used in the experiment. When photographing, a silicon diode was used to check the synchronization of flash lamp and laser.
had several emission lines instead of a pure continuum in the region of interest. It was found that a high pressure (1.8 bar) of Ar reduced the afterglow (Figure 2) whereas the first spike did not change its energy content between 400 and 1800 mbar. The temporal resolution was determined by the length of the flash. Discharging of 0.1-pF capacitor (charged to 35 kV, 60 J) through the low inductance circuit produced a 400-ns half-width in the 174-nm region while in the visible, the emission was much longer. The resolution of the "pumped" vibrational structure was improved to 150 ns by choosing a correspondingly short overlap time for UV and IR radiation (Figure 2). The vibrational excitation laser was a homemade transverse discharge atmospheric pressure C 0 2laser. It emitted 4-8 J in each employed line with 25% of the energy contained in the first spike of 70-11s half-width and the remainder in a tail of 3 ps. This was the case for all lines except P40 where the spike was 120 ns long and had 30% of the energy. When two cylindrical lenses were used the beam was enlarged in one direction and focussed through
W
J 1010
1030-
'
I
1050
1070
cm-I
1030
Figure 3. COz laser lines employed in the flash spectroscopy experiment. For comparison, a spectrometer spectrum5and dissociation probabilities P, are also shown (0from ref 8, 0 from ref 9; the latter are apparent P, which are about 30% smaller than primary yields owing to recombination of the fragments. P,, of %F,I has been constructed by muklplying P , of %F31 by the selectivities measured in focused geometry.'2 ~ h e s eselectivities agree with those in a parallel beam of 2 J/cm2 at P40 and P36.e
the side window of the cell, making a spot size of 7 X 95 mm2 (inner length of the cell 90 mm). A factor of 2 increase of energy density was attained by reflection of this beam at the back w d . The cell was made of copper. Two far-UV quartz windows and one germanium window were attached with silicon rubber. The cell was cooled to 180 K by a cryocooler with circulating helium. It was surrounded by a vacuum jacket for thermal insulation. It contained, apart from another germanium window, a bandpass filter with maximum transmission of 15% at 172 nm and half-width 15 nm to reduce long-wavelength stray light and to suppress near-UV photolysis of CF31. A quartz lens of focal length 4 cm at 174 nm was also mounted on the cell.
Results and Discussion Figure 3 shows the C02 laser lines used, together with the spectrum in the linear absorption regime. Dissociation probabilities at 2 J/cm2 are also shown to give an idea of the efficiency of excitation. Figures 4-7 show densitometer traces of the UV spectra of CF31,without and with irradiation by selected C02 laser lines. Ground-state CF31 (Figure 4) exhibits very short progressions of the three nondegenerate modes, as indicated by the ladders in the figure; each ladder rung can be the starting point of a new ladder, and for the u3 ladders of this type the quantum numbers have been omitted. The vibrational quantum numbers of the upper and lower electronic states are given as upper and lower indices of the number of the mode. Each ground-state absorption has a hot band precursor, 50 cm-' (=0.16 nm) to longer wavelengths, which starts from the states u3 = 1 (energy 286 cm-') and u6 = 1 (265 cm-'1. The population of these states a t -90 "C is 35% of the ground state, in rough agreement with the relative intensity of the precursor bands. A t room temperature the absorption cross section (measured by a multiplier instead of the photographic fii) of the bands 0 and 1, (and therefore probably also of 1J is 5 X cm-2. At 180 K, due to spectral narrowing, it is calculated to be cm-2. This should be large enough to observe relative populations down to 1% at a pressure
The Journal of Physical Chemistry, Vol. 86, No. 5, 1982 733
Flash Spectroscopy of Multiphoton Excited CFJ TABLE I :
Vibrational Frequencies ( c m - ’ ) in t h e Lower ( v ” ) and Upper ( u ’ ) Electronic States and Degeneracies d
~-
1
2
1075.2 969 106
1 743.0 682 61
i
1
di
ui”c vi:d vi
- vi
a This work.
,
Split by Jahn-Teller interaction.
3
1 286.2 231 55 Reference 5.
4
5
6
2 1187.2
2 543.1 458 f 15a9b 8 5 r 15
2 262 221; 214b 41; 4 8
Reference 10.
t
OD
Figure 6, UV spectrum of CF,I excited at 1055.6 cm-’ (P10). Only collislonaliy populated states are found. See also caption for Figures 5 and 4. The arrow indicates the expected absorption of the first populated state: 0.4 mbar. 1.0 J/cm2.
Flgure 4. Densitometer traces of the UV absorption spectrum of CF,I at -90 OC, without the C02 laser. OD, the optical density of the photographic film, increases with transmission of cell: (1) empty cell; (2) 15 pbar of CF,I; (3) 0.7 mbar of CFJ. The assignments are from ref 11, where a slightly better resolved spectrum at r t ” temperature can be found. The numbers glve the numbers of mode, their lower and upper indices give the corresponding vibrational quantum numbers in the lower and upper electronic states.
t
OD
A P 40
1
OC
175
nm
1 0
Figure 7. UV spectrum of CF,I excited at 1027.4 cm-i (P40). See caption to Figures 5 and 4: 0.8 mbar, 0.6 J/cm2.
Flgure 5. UV spectrum of CF,I excited by the COPlaser at 1081 cm‘ (R24). Each ladder connects transitions from a single inltlal state plus its hot band precursor(s). Only the first transition of each ladder is indicated; in the ensuing members, the quantum numbers vary as in Figure 4. Parentheses indicate transitions from collisionally populated states: (1). 0.15 mbar, 0.4 J/cm2; (2) 0.7 mbar, 0.1 J/cm2; (3) 0.7 mbar, 1.4 J/cm2 (spike plus tail).
of 0.1 mbar in the 9-cm cell. Pressures as low as that would be desirable to prevent collisional redistribution of energy which tends to blur out the spectra. In fact, when the flash lamp was fired 5 ps or more after the laser, only a structureless continuum was observed. Unfortunately, much higher pressures (to 0.8 mbar) were necessary to observe
the laser-produced vibrational structure. The reason was probably that pumped states were observed only during about 150 119 (see the timing in Figure 2), whereas the flash lamp spike had about 800 ns at base. Due to the high pressure, states were also observed which were clearly populated by collisions (e.g., the states up and u3 which are far off resonance with the laser); they are enclosed in parentheses in Figures 5-7. The gas kinetic collision rate at 180 K is 13 w9-l mbar-’. Thus at the highest pressure employed (0.8 mbar) nearly 80% of the excited molecules have experienced a collision during the main observation time of 150 ns. An additional thermal component is contributed to the spectra by the weak but long tail of the flash lamp. However, the extent of the collisional redistribution in Figures 5-7 is only moderate as demonstrated by the strong spectral changes with increased delay. Furthermore, in the discussion the absence of population of some levels will be emphasized. Obviously, such an absence is information about the collisionless excitation, too. Each populated state has a spectrum similar to that of the ground state (Figure 4),but shifted by differences in
734
The Journal of Physical Chemistry, Vol. 86, No. 5, 1982
the vibrational frequencies (Table I). The Franck-Condon factors are similar for similar magnitudes of changes in quantum numbers. The transitions from each single initial state plus their hot band precursors are connected in Figure 5-7 by ladders, which are contractions of the ladders in Figure 4; only the first transitio’n of each ladder is denoted. The fact that hot band precursors occur with the same relative intensity in Figure 4 and Figures 5-7 means that the laser excites the infrared fundamental and (nearly coinciding) hot bands with equal probability. When the laser is tuned to R24, a frequency within the infrared ul R branch and the 2u: Q branch, one observes bands (denoted by 52 in Figure 5) shifted by 170 cm-l from the ground-state spectrum. They are appreciably weaker when the laser was on the R14 to R10 lines (not shown) which are not as strongly in resonance with the infrared 2u5 transitions as in R24. Thus they must obviously be assigned to transitions starting from the 2u5 states. The frequency u5 of the upper electronic state (Table I) has been derived in this way. These bands are broad compared, for example, with the nearby l1bands. Their width is probably due to unresolved Jahn-Teller splitting. The Jahn-Teller effect couples electronic and nuclear motion if both the electronic state and the vibrational state are degenerate. Whereas the first condition is met for the C state, among the vibrational states only the sublevel 2u2, is degenerate, whereas 2u: is not. Obviously, the 2ui state is also populated by the laser. This lends further credence to the assignment of the infrared 0 2ui transition at 1083 cm-’ and to the postulate that this band has an oscillator strength not very much smaller than 0 u1 and 0 --+ 2 4 , although the corresponding fundamental 0 u5 is much weakera5The high intensity of all three transitions implies that very many transitions between sublevels of the multiplets of overtone and combination levels of ul, 2u:, and 2 4 also have high transition probabilities. This fact facilitates multiphoton excitation.’ Figure 5 shows that when CF,I is excited by the laser line R24, the levels ul, 2u5, 2ul, and u1 + 2u5 (and possibly 4u5, which is difficult to detect) are populated. Their energies are one or two times the photon energy hub They are already observed a t a fluence of 0.01 J/cm2. No discrete band of any level a t 3hvLor above can be observed in Figure 5, although an appreciable fraction of CFJ dissociates under the high-energy conditions. Instead, a weak unresolved background appears at long wavelengths where the higher states should absorb. In contrast to that, the absorption bands of 3ul and 2u1+ 2u5 (energy 3ha) can clearly be discerned above the background when the molecules are irradiated by R10. This difference is certainly due to the fact that resonant ul and 2u5 transitions exist for R10 up to 3huL,but only to 2huLfor R24 (see next section). Apart from the 3huL features, the spectra for pumping by R10 to 14 are very similar to the R24 case. Therefore they are not shown. But what happens when the u1 and 2v5 transitions are out of resonance after the second or third step, respectively? To answer this question it is instructive to study the excitation by P10 (Figure 6) or P16 (not shown, similar to Figure 6). Neither frequency finds any single-photon resonance at 180 K, where the rotational contour is even narrower than in the 220 K spectrum shown in Figure 3. Nevertheless, at the employed intensities they are absorbed as dissociation was observed in the experiment: after a few shots the CF31 pressure had dropped noticeably. Obviously, the first step is a direct two-photon transition, most probably to ul + uz + u3 and u2 + u3 + 2u5 which have suitable energies. However, these states cannot be detected
-
--
Fuss
in Figure 6 (see arrow). Instead, apart from a background, bands are detected starting from states (u3, u2, u l ) which are clearly out of resonance and which can be populated only by collisions. Obviously, the two-photon resonant states are depleted at a similar or faster rate than they are populated. The first step is a bottleneck. When it is overcome, suddenly a whole ladder of states is populated whose UV spectra superimpose to give a nonanalyzable background. A similar bottleneck probably occurs after the second step for R24 or third step for R10 to 14. Note in this context that the dissociation yield at the R lines has been found6 to depend strongly on the pulse length (or intensity), as expected when a direct multiphoton step controls the rate of excitation. Similar features are observed when the molecules are excited by P40, in the wing of the u2 u3 Q branch (Figure 3). The combination mode uz + u3 itself is nondegenerate and no near-degeneracy occurs either. Thus, one expects excitation of the states uz + u3 (by a Q-branch transition) and possibly 2uz 2u3 (by an ensuing R-branch transition), but no higher excitation since the mode goes out of resonance. This is exactly what is found (Figure 7). Since the dissociation yield in this wavelength region again depends strongly on the temporal substructure (on the intensity) of the laser pulse, the next step will again be a direct multiphoton transition. In this context it is instructive to look to the dissociation of 13CF31. Figure 3 shows that its maximum occurs at P36, where the small signal spectrum has an absorption minimum. This wavelength coincides, however, with a direct two-photon (AJ = 0) absorption to v1 u2 u3. For a few initial rotational states, this level can also be reached by A J = 1 and 2 transitions by pumping with P40; and for this wavelength a similar two-photon transition leads from u2 + u3 (populated by single-photon absorption) to v1 + 2(u2 + u3). The only similar transition of 12CF,I at P40 is from 2(uz + u 3 ) (populated by two single-photon steps and identified in flash spectroscopy) to v1 + 4(uz + us) (three photons). At this point it is helpful to introduce level diagrams. They are displayed in much the same form as in ref 1 or 3.
+
+
+ +
Level Diagrams The energies (modulo 1075.2 or 1028.0 cm-’) of overtone and combination levels of ul, uz + v3, and 2v5 are shown in Figures 8 and 9. Thus the figures only show the anharmonic defects; harmonic positions would lie on straight lines. The ordinate (n)is related to the quantum numbers as given in the caption of Figures 8 and 9; it is also a step number serving for comparison of level energies with n times the photon energy. The overtone levels can be connected by parabolas (see especially the levels n(uZ+ u3) in Figure 8) whose curvatures are determined by the anharmonic constants. Multiples of the energies of COz laser quanta appear in the figures in the form of straight lines. (6) I. N. Knyazev, Yu. A. Kudriavtsev, N. P. Kuz’mina, V. S. Letokhov, and A. A. Sarkisian, Appl. Phys., 17,427 (1978);I. N. Knyazev, Yu. A. Kudriavtsev, N. P. Kuz’mina, and V. S. Letokhov, Sou. Phys. JETP, 49, 650 (1979). (7) Yu. A. Kudriavtaev and V. S. Letokhov, Chem. Phys., 50, 353 (1980). (8) M. Rossi, J. R. Baker, D. M. Golden, Chem. Phys. Lett., 65, 523 (1979). (9) W. Fuss, unpublished results. (10) G. Herzberg, “Molecular Spectra and Molecular Structure”, Vol. 3, Van Nostrand, Toronto, 1966. (11) L. H. Sutcliffe and A. D. Walsh, Trans. Faraday SOC.57, 873 ( 1961). (12) M. Drouin, M. Gauthier, R. Pilon, P. A. Hackett, and C . Willis, Chem. Phys. Lett., 60, 16 (1978). (13) According to a private communication by A. A. Makarov, the observed anharmonic shift of vg is more like 60 cm-I instead of 100 cm-’ as suggested by an inaccurate figure in ref 4.
Flash Spectroscopy of Multiphoton Excited CF,I I
1
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Flgure 8. Level energies E, - n(1028.0 cm-') close to n times the photon energy. The energies have been calculated from the frequencies and anharmonic constants of ref 5, including Ferml resoI= nance; vibrational angular momentum: (0)I = 0,(+) I = 2 , (0) 4. The curves correlate the levels 2ul (n - 2Xu2 u,) (x); u1 (n - 1 ) (u, u,) (y); 2(v2 v,) (2). The thick lines represent multiples of photon energies for the indicated laser lines. For several selected initial rotational states, the change of rotation energy (= 2mAJ for a parallel transition) has been subtracted from these lines (thin solid and broken lines); therefore a P(R) branch transltlon is represented by a steeper (flatter) slope than the central line, which itself nearly coincides with a Q-type transition.
+
+
+
+
+ +
. . . . ..
st.
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I
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The longer their wavelength, the stronger they are inclined to the left-hand side. Instead of adding the rotational to the vibrational energies, the change of rotational energy = BBJAJ, has been subtracted in each step, E p t from the photon energy. Because A J = 0, f l , this would give rise to three lines of different slope for each vibrational state and for a given initial rotational state. The line for consecutive Q branch transitions would approximately coincide with the straight line representing the laser frequency; P and R transitions would appear on ita right- and left-hand side, respectively. Only those lines are shown which represent the rotational-vibrational transitions best in resonance. Thus the excitation path in Figure 8a (P40) shows a two-step QR resonance followed by a A J = -3 three-photon transition. The second and third steps are possible only for selected (but identical) rotational states. In a similar way Figure 8b for 13CF31shows the twophoton A J = 0 resonance of v l + u2 + v3 with the P36 wavelength. The same state can be reached via u2 + u3 (consecutive R and P transitions) or via v1 (consecutive P and R transitions) for certain initial rotational states of very high J. For smaller J values, such transitions are still near resonance, a fact which certainly contributes to the importance of direct two-photon transitions in this and the other cases. The same figure also shows excitation by P40 a A J = 1 single-photon step followed by a A J = 1 twophoton transition. Direct two-photon A J = 0 transition excited by P10 and P16 are shown in Figure 9a. From this figure it is obvious that once this bottleneck is overcome, the molecule can easily continue with single-photon transitions ( A J = 0, f l , not indicated in the figure). This is as postulated above to explain why the v1 up u3 state was not observed in flash spectroscopy. Excitation by R24 is shown in Figure 9b. A large number of rotational states can be transferred to the 2uE state via Q branch transitions. Different rotational states are
-60 -30 E n - n ,1075 2cm-1
-90
-120cm-1
30
E n - n 1028Ocm-'
!
\.,
1
C
1
,
-120cm-1
I
I.
1
-90
1
,
I
I
,
I
-60 -30 En- n , I075 2cm-1
,
\A! 1 ,\y0
Figure 9. Level energies E, - n(1075.2 cm-') close to n times the photon energy. The states of vibrational angular momentum I> 0 have been omitted for n = 5 and for the lower energies of n = 4. The energies have been calculated from the frequencies and anharmonic constants of ref 5, including Fermi resonance: (0)I= 0 , (+) I = 2 , ( 0 )I = 4, ( + ) I = 6 , (0)I = 8. The curves correlate the levels n u l (a); 2nv0 (b); 2nu2" (c); (n - 1)u, (up u,) (d); (n - 2)u1 2(u2 4 (e). The thick lines represent multiples of photon energies for the indlcated laser lines. For several selected initial rotational states, the change of rotation energy (= 2 m A J for a parallel transition) has been subtracted from these llnes (thin s o l i and broken lines); therefore a P(R) branch transition is represented by a steeper (flatter) slope than the central line, which itself nearly coincides with a Q-type transition.
+ +
+
+
subsequently pumped to different n = 2 levels, via R-type transitions. Certain rotational states can also be excited to v1 (R branch) and then to u1 + 2125 (broken line). The latter transition is perpendicular (AK# 0, AZ = 2). The nearly resonant transition to the next level, 6~65,is too weak to occur as it would imply a change of the vibrational angular momentum 1 by 4 units. For the same reason ul + 4vi cannot be populated. Thus, no population at any n = 3 level is expected, in agreement with the results of flash spectroscopy. However, at R10 several n = 3 levels can be populated (Figure 9c), whereas no strong transition leads to any n = 4 level. This is again in agreement with the flash spectroscopy.
Conclusion Flash spectroscopy directly detected the levels populated by the first one to three photons at 1027.4 cm-' and in the range 1050-1081 cm-'. Only levels are found which are (probably) in resonance within about i0.5 cm-' (their energy is often not better known) and which are connected by intense infrared transitions. Although "background states" achievable by a large change of vibrational quantum numbers are available in the range 1050-1081 cm-', they were not detected. Indirect evidence also suggested that transitions to background states (to the
J. Phys. Chem. 1982, 86, 736-739
736
“quasicontinuum”) are less important at least up to n = 5 than direct two- or three-photon transitions. The evidence was as follows: (1)excitation by P10 or P16, where the first step has to be a two-photon transition, generates similar effects as observed a t other wavelengths in flash spectroscopy; (2) the dissociation yield strongly depends a t all investigated wavelengths on the laser intensity, indeed stronger than expected for an explanation invoking hole burning in the rotational contour; (3) the carbon isotope selectivity around P40 is determined by a bottleneck which apparently consists for 13CF31in a direct two-photon transition in the first step and for 12CF31in a (more difficult) direct three-photon transition in the third
step. It would, however, be harder to understand why absorption to the background states (quasicontinuum) should be more difficult in the third step in 12CF31than in the first step for I3CF3I. Thus, indirect evidence indicates the nature of the bottleneck transitions, up to the fifth photon. It is desirable, however, to observe these and higher states directly. This would be feasible with improved sensitivity of flash spectroscopy. Acknowledgment. I thank J. Hartmann for his expert assistance in setting up and performing the experiment, and K. L. Kompa for his support and discussions.
Vibrational Energy Transfer in Laser-Excited COF,. Intermediate Mode v4
Infrared Fluorescence from the
R. K. Bohn,+ K. H. Casleion,z Y. V. C. Rae,§ and 0. W. Flynn” Department of Chemistry and Columbia Radiation Laboratory, Columbia University, New York, New York 10027 (Received: July 23, 1981; I n Final Form: October 13, 1981)
Infrared fluorescence from the v4 (C-F asymmetric stretch) mode of COF2has been observed following laser excitation of the v2 (C-F symmetric stretch) mode. A kinetic anomaly exists in that v4 appears to fill more slowly than it empties leading to a double exponential rise in v4 fluorescence. The slowest observed “intermode” relaxation process in COF2requires approximately 475 collisions while overall vibration-translation/rotation relaxation requires approximately 2800 collisions.
Introduction In a previous study1 COFz was shown to absorb C 0 2 infrared laser radiation in the v2 mode (C-F symmetric stretch). The overtones of the v2 mode were found to be rapidly populated by the ladder-climbing process 2COFq(v,) --* COFz(2vz) + COFz(0) (1) Since the 2v2 level is Fermi mixed with the v1 carbonyl stretch state, strong IR emission is observed from 2v2 0 at X 5 pm. For certain C02 lines, population in the 2v2state can be directly produced by hot-band pumping of the v2 2v2transiti0n.l Because of the rapid coupling of v2 and 2u2 via process 1, which requires an average of 30 gas kinetic collisions to reach steady state, fluorescence from 2v2 simply reflects the changes in population of the v2 state for processes slower than (1). Though in principle the decay of the vl, 2v2 levels could reflect direct relaxation of these states to other nearby levels, in practice the vq level, which has typically 100-1000 times as many molecules as 2v2, vl, acts as an efficient “buffer” replenishing any direct loss of population from these states via the ladder climbing event (1). There are several features of vibrational energy transfer in COF2 which distinguish it from other polyatomic molecules, mostly derivatives of CH4,2that have been investigated in detail. Since COF2 is completely composed of heavy atoms, most of the fundamental vibration frequencies are relatively low frequency rather than clustered
-
+
-
Department of Chemistry, University of Connecticut, Storrs, CT. * U S . Department of Energy, Morgantown Energy Technology Center, Morgantown, WV 26505. 5 Indian Institute of Technology Kanpur, Kanpur 20806, India.
0022-3654/82/2086-0736$0 1.2510
into regions of high and low frequencies. The large moments of inertia of COF2 are expected to reduce the importance of rotational motions in the vibrational energy transfer processes3and to allow for direct collisional energy transfer between the fundamentals ( u = 1 levels) of most modes. Relatively strong fluorescence may be observed from a t least four different states (2v2,vl) at 5 pm, v4 at 8 pm, V g at 13 pm, and v3 and/or vg a t 16-17 pm. Thus, the populations of these states may be independently monitored as a function of the time after pulsed laser pumping. COF2is a tetraatomic molecule with relatively dense energy levels whose energy transfer properties should more closely approximate those of larger molecules. Stretch-stretch energy transfer occurs rapidly in CH,, 11 f 3 collisionss, for the symmetric-asymmetric stretch transfer which is endothermic by about 100 ~ m - l .Since ~ COF, contains no hydrogen atoms, determination of stretch-stretch energy transfer rates will indicate whether heavy atom molecules behave differently. The 8-pm fluorescence from the v4 state of COFz is particularly interesting because its growth after pulsed laser pumping is characterized by two exponential rises. This behavior is rather unusual and has not been fre(1) K. H. Casleton and G. W. Flynn, J. Chem. Phys., 67,3133(1977).
As in this reference, a collision diameter of 6 A and a temperature of 296
K were used to convert energy transfer rates t o collision numbers in the present paper. (2) E. Weitz and G. Flynn, Adu. Chem. Phys., 67,185-235 (1981). A notable exception is the energy transfer pathway in OCS, M. Mandich and G. W. Flynn, J. Chem. Phys., 73, 1265 (1980). (3)C. B. Moore, J. Chem. Phys., 43, 1265 (1965). (4) P. Hess, A. H. Kung, and C. B. Moore, J. Chem. Phys., 72,5525 (1980).
0 1982 American Chemical Society
