J . Phys. Chem. 1984,88, 1283-1284
1283
Rovibronic Emission Locked Fluorescence Excitation Spectroscopy. A Selective Probe for Coriolis Perturbation in Rovibronic Levels of S, H,COt Eric C. Apel and Edward K. C. Lee* Department of Chemistry, University of California, Irvine, California 9271 7 (Received: December 27, 1983)
The fluorescence excitation spectrum of the H2C0 S1 So transition in the 2;4; region has been taken with the fluorescence band locked onto a specific rovibronic emission feature. This is a very useful spectroscopic probe of Coriolis perturbation. It permitted the assignment of a new vibronic transition, 2:4;6;, which overlaps with the stronger vibronic transition, 2;4:, and borrows intensity through b-axis Coriolis coupling with 2I6l. +-
Vibrational-state mixing is the key to the understanding of intramolecular vibrational redistribution in polyatomic molec u l e ~ . ’ - ~In molecular spectroscopy, it is often manifested by Fermi and Coriolis perturbations, the former being rotation independent and the latter being rotation dependent. Radiationless processes in small polyatomics have shown some significant rotational-state dependence as well as vibrational-state dependence.In order to obtain detailed experimental information on rotation-vibration interaction in the electronically excited states of polyatomic molecules, we have recently employed the technique in which the fluorescence emission (FEM) spectrum is wavelength analyzed at rotational resolution and then the fluorescence excitation (FEX) spectrum is taken with the emission band-pass locked onto a given terminating rovibrational level of the ground electronic state. A remarkable utility of this rovibronic emission locked fluorescence excitation spectroscopy is illustrated below. The A1A2-W1Al electronic transition in H2C0 is vibronically allowed B-type bands are about twice as strong as C-type bands, and A-type bands are at least 1 order of magnitude weaker than C-type bands.I0 The experiments were carried out under collision-free conditions as described e l ~ e w h e r e .An ~ FEX spectrum (of total emission) taken in the ‘R4 (J’? subband region of the 2A4: band (B-type) with a pulsed, Nd:YAG pumped dye laser (Quanta-Ray, DCR-l/PDL-1, -7 ns, -0.08-cm-I fwhm) is shown in Figure IC. The wavelength-analyzed emission spectra taken from the excitation of two rotational lines marked by a filled circle and an open circle (in Figure IC) are shown in Figure 2, a and b, respectively. The rotational line intensities of the emission to the u,” = 1 (al) level from thefilled-circle excitation has a distributih expected for a B-type band; the upper state can be assigned as the J’= 10, K’= 5 level of 2143(origin = 30340.15 cm-I).l1 The u2” (‘R) emission locked FEX spectrum is shown in Figure la, and the rotational transitions assigned here are consistent with those of ‘R4 (J”), 2h4: given by Sethuraman, Job, and Innes.12 On the other hand, the emission spectrum from the open-circle excitation (see Figure 2b) is distinctly different: (i) the virtual absence of the u2” (al) emission, (ii) the presence of the ug/) (b,) emission with rotational line intensities characteristic of a perpendicular band, giving the upper state assignment of J’ = 9 (with the experimental uncertainty of *l), K,’ = 5 of an a2 vibration, and (iii) the presence of the 2v/ (al) emission with a resemblance to a perpendicular rotational bandshape, giving the upper-state assignment of J’= 9 ( f l ) , K,’ = 6 of a b2 vibration. The 2u/ (PP)locked FEX spectrum is shown in Figure 1b, and the rotational transitions have been assigned as the qR, (J”) subband of 2;4;6;. This assignment is based on rotational analysis and a supersonic-jet studyI3 in which we have found two new vibronic transitions, 2A6; (origin N 30255 cm-I; T N 42-68 ns) and 2;4;6: (origin 30370 cm-I; T 20 ns) in addition to the
‘Presented at the International Conference on Radiationless Transitions, Newport Beach, CA, Jan 3-7, 1984.
0022-3654/84/2088-1283$01.50/0
well-established 2A4; ( T I15 ns). The zero-order =(b2) level can mix with the zero-order 2]4161 (a2) level by b-axis Coriolis coupling as shown in Figure 3. The rotational quantum numbers in ii and iii above differ by AK; = i l . The observed vibrational energy gap between the two eigenstates 2I6I and 2l4l6’ is 115 =k 10 cm-I. The A J ’ = 0, AK,I = -1 resonance should occur at K,’ N 7 (if Aeff 3: 9.0 cm-I and B 1.06 cm-’ are assumed), and thus the rotational subbands studied here (KL = 5 and 6) are clearly below this resonance. The consideration of the observed vibronic origins of 2h6; and 2;4;6A together with the above two factors supports (a) the rotational subbands shown in Figure l b to be qRS(J’? of the 2;4;6; (214‘61 parentage) which gains intensity by b-axis Coriolis coupling with 2161, (b) the vg/l emission in Figure 2b to be due to the 2I4l6l zero-order component, and (c) the 2vql’ emissionI4 to arise from the =zero-order component. The vibrational mixing of 2161 and 2l4l6l by b-axis Coriolis coupling is substantial. It should be noted that w ( K , ’ = 5 ) does not Coriolis couple with (K,’ = 6) to any significant extent as is evidenced in Figure 2b. However, Figure 2a is indicative of prominent C-type Coriolis mixing of E ( K ; = 5) with w ( K , ’ = 4) which occurs because U’,, N 30 cm-I. In this spectrum a-type Coriolis mixing of 2147 (K,’ = 5 ) with 2161(K; = 5) or with 3161 (KL = 5) occurs to a lesser extent because of an unfavorable energy gap N 85 cm-l). The rovibronic emission locked FE,X spectra (Figure 1, a and b) can mostly account for the complexity of the. total FEX (1) K. F. Freed and A. Nitzan, J. Chem. Phys., 73, 4765 (1980). (2) C. S. Parmenter, J. Phys. Chem., 86, 1735 (1982). (3) R. E. Smalley, Annu. Rev. Phys. Chem., 34, 129 (1983). (4) (a) N. L. Garland, E. C. Apel, and E. K. C. Lee, Chem. Phy.s. Lett., 95, 209 (1983); (b) N. L. Garland and E. K. C. Lee, ibid., 101, 573 (1983); Faraday Discuss. Chem. Soc., 75, 377 (1983). (5) K. Y. Tang, P. W. Fairchild, and E. K. C. Lee, J . Chem. Phys., 66, 3303 (1977). (6) (a) See, for a review, E. K. C. Lee and G. L. Loper in “Radiationless Transitions”, S. H. Lin, Ed., Academic Press, New York, 1980, p 1; (b) K. Shibuya, P. W. Fairchild, and E. K. C. Lee, J . Chem. Phys., 75, 3397 (1981). ( 7 ) See, for a review, C. B. Moore and J. C. Weisshaar, Annu. Reu. Phys. Chem., 34, 525 (1983). (8) E. Riedle, H. J. Neusser, and E. W. Schlag, J . Phys. Chem., 86,4847 ( 1982).
(9) H. Stafast, H. Bitto, and J. R. Huber, J . Chem. Phys., 79,3660 (1983). (IO) S. J. Strickler and B. J. Barnhart, J . Phys. Chem., 86, 448 (1982). (11) V. A. Job, V. Sethuraman, and K. K. Innes, J . Mol. Spectrosc., 30, 365 (1969). (12) V. Sethuraman, V. A. Job, and K. K. Innes, J . Mol. Spectrosc., 33, 189 (1970). (13) E. C. Apel and E. K. C. Lee, unpublished work. The experimental setup used is similar to that in ref 4a. (14) In Figure 2b, the separation between the ‘R and PP lines of the 2u4” emission appears too small for the values of K” = 5 and 7, respectively. This is probably due to the a-axis Coriolis coupling between two zero-order states of So, 4and & and it is appropriate to consider the emission to be transitions to “mixed” rovibrational levels of 4>parentage with K / = 5 and 7. The other less probable alternative is that these lines are transitions to mixed rovibrational levels of 4,6, parentage with K” = 3 and K ” = 5 from the J’ = 9, K’ = 4 level of 446’; in the absence of IR or other electronic emission data it is inconclusive.
0 1984 American Chemical Society
Letters
1284 The Journal of Physical Chemistry, Vol. 88, No. 7, 1984 30398 I
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vex ( c m - ' ) Figure 1. Fluorescence excitation (FEX) spectra of the H2C0 A'A2 RIAl transition near the 2;4: 'R4(J'? subband. The pressure of H2CO was 400 mtorr at room temperature. (a) FEX taken by locking the emission band-pass at 3490 A (see Figure 2a); (b) FEX taken by locking the emission band-pass at 3578 A (see Figure 2b); (c) FEX taken by
viewing the total emission.
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Figure 2. Wavelength-resolved fluorescence emission spectra obtained by exciting two different rotational levels by (a) 'R,(9) 2h48 transition (0) and (b) 9Rs(S) 2:4;6; transition ( 0 ) indicated in Figure 1.
spectrum (Figure IC). Spectra such as these will be extremely valuable in studies of Coriolis interaction in the excited electronic states of polyatomic molecules, complementary to other experimental techniques.15-" (15) C. M. L. Kerr and D. A. Ramsay, J . Mol. Spectrosc., 87, 575 (1981).
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Figure 3. Vibrational energy level diagram of nine vibrational levels within 150 cm-' of 2i43 and 2I4l6l eigenstates. Different Coriolis cou-
pling schemes are shown for each pair of zero-order vibrational symmetry species. Our present study and an earlier study4J3in our laboratory on the vibLationa1 mixing involve two different regions of SI HzCO(AiAz),AElyjb= 2200 and 3000 cm-', where the average energy-level separations are approximately 50 and 20 cm-', respectively. In each study, we have taken measurements of rotationally resolved FEM spectra from a single rotational level excitation under collision-free conditions. In the ELb= 2200 cm-* region, extensive vibrational mixing of two zero-order states, 214161 and w b y b-axis Coriolis coupling, and =and =by c-axis Coriolis coupling (AJ' = 0 and AK,' = =tl) has been observed. The a-axis Coriolis mixing (for AJ' = 0 and AK,I = 0) between 2143and =(or 3'6') was observed to be less pronounced due to an unfavorably large vibrational energy difference. In the ELb N 3000 cm-' region," an extensive vibrational mixing among three zero-order states has been observed: between w a n d z b y a-axis coupling due to a favorable vibrational energy gap (-2.7 cm-I), ' between 1'41and by b-axis coupling, and between 5' and 1 by c-axis coupling. A further study at higher values of E/vibshould reveal even greater degrees of vibrational mixing.
1'
Acknowledgment. This research has been supported by the National Science Foundation (Grant CHE-82-17121). (16) E. Riedle, H. J. Neusser, and E. W. Schlag, J . Phys. Chem., 86,4847 (1982). (17) (a) P. H. Vaccaro, J. L. Kinsey, R. W. Field, and H.-L. Dai, J . Chem. Phys., 78, 3659 (1983); (b) D. E. Reisner, Ph.D. Thesis, Massachusetts Institute of Technology, Cambridge, MA, 1983.
