UV-IR double-resonance spectroscopy of jet-cooled propynal detected by the fluorescence dip method

16 December 1994

CHEMICAL PHYSICS LETTERS ELSEVIER

Chemical Physics Letters 231 (1994) 64-69

UV-IR double-resonance spectroscopy of jet-cooled propynal detected by the fluorescence dip method Th. Walther, H. Bitto, T.K. Minton, J. Robert Huber Physikalisch-Chemisches Institut der Universitiit Ziirich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland Received 12 September 1994

Abstract Pulsed UV-IR double-resonance spectroscopy was implemented to demonstrate IR absorption spectroscopy in electronically excited polyatomic molecules by fluorescence dip detection, using jet-cooled propynal (HCCCHO) excited to the first excited singlet state S1 as an example. This method opens the possibility to study the spectroscopy and dynamics of rovibronic states which are not accessible in one-photon transitions or are non-fluorescing.

1. Introduction Double-resonance spectroscopy is a useful and powerful tool for the investigation of molecular structure [ 11. With the advent of pulsed and tunable lasers this spectroscopic method has been extended from the frequency to the time domain providing also direct information on molecular dynamics. In recent years a variety of pump/probe techniques and detection schemes have been introduced of which we shall focus on the UV-IR double resonance method, more specifically, fluorescence dip spectroscopy with regards to polyatomic molecules [ 21. The two-step excitation of this technique, as illustrated in Fig. 1, involves population of a vibronic state by a short and intense UV pulse followed by an IR pulse which is tuned over the absorption of higher rovibronic states. In the present work the detection of the second (IR) absorption process is achieved by the emission from the intermediate state, the intensity of which is reduced whenever the IR laser is tuned into a resonance. Fluorescence dip spectroscopy, similar to ionization dip spectroscopy [ 3 1, was introduced by Ebata

states in NO [ 41. Later Ito and co-workers used the technique to investigate Rydberg-Rydberg transitions in order to explore the mechanisms of autoionization processes in molecules such as DABCO, ABC0 and pyrazine [ 5-71. Parallel and complementary to fluorescence dip spectroscopy, they employed multi-photon ionization detection and demonstrated the versatility of fluorescence dip spectroscopy below the ionization potential of the molecules studied. Recently, Ito and co-workers applied fluorescence dip and phosphorescence dip spectroscopy to localize higher electronic transitions in pyrazine [ 8 1. Using UV and near-IR/visible lasers these workers were able to establish the positions of two theoretically predicted electronic states, namely the singlet state S, ( ‘BZg) and the triplet state T, ( 3B29). Since the measured resonance signals showed linewidths of several 1000 cm-’ the detection of vibrational or rotational structures was not feasible. In contrast to these previous studies, we report in this Letter the first application of fluorescence dip spectroscopy to vibrational transitions within an et al. to study Rydberg

0009-2614/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDIOOO9-2614(94)01224-5

Th. Watther et al. /Chemical

H--$X-C

//O +H

Fig. 1. Excitation scheme of UV-IR double resonance experiments. An UV pulse populates a rovibronic level in the first electronically excited singlet state St and, subsequently, the absorption of a delayed IR pulse is measured by the decrease of the fluorescence from that state. On the top the structure of propynal with the C-H stretching modes v1 and v2 is shown.

electronically excited state and illustrate applications of the method to the study of the spectroscopy and dynamics of polyatomic molecules with the molecule propynal.

2. Experimental The experiments were carried out under isolatedmolecule conditions in a pulsed supersonic jet expansion (0.7% propynal in 1.6 bar neon). Single rovibronic transitions in the first excited singlet state S, of propynal (see Fig. 1) were selected with an UV pulse generated by an excimer (Lambda Physik, LPX 200) pumped dye laser (Lambda Physik, FL2002). The dye laser produced pulses at 2% 380-350 nm (dyes BiBuQ, BMQ ) with a duration of = 20 ns and a bandwidth of 0.04 cm-‘. The Si-So fluorescence signal was recorded as a function of the frequency of the time-delayed IR pulse, which was generated by a home-built optical parametric oscillator (OPO ) , pumped by an injection seeded Nd:YAG laser (Quantel, YG-680). The OPO is similar in design and specifications to that of Minton et al. [ 9 1 and has been

Physics Letters 231(1994) 6469

65

described in Ref. [ 10 1. For the present study the bandwidth of the OPO idler radiation was varied between 3 (injection seeder off ), 0.8 (injection seeder on) and 0.03 cm-‘. In order to achieve a line~dth of 0.03 cm-’ an Ctalon was placed into the cavity for selection of two longitudinal cavity modes allowing continuous scans over a range of up to 5 cm-‘. The intensity of the detected fluorescence (see Fig. 1) is reduced whenever the IR laser is tuned to a resonance either upwards or downwards in energy from the UV pumped state. As has been shown in the case of stimulated emission pumping, the distinction between the two transitions can be achieved when rotational resolution for both the UV and IR excitation is possible [ 111. The temporal delay between the WV and IR laser pulses has two purposes, first to avoid any coherent effect caused by a temporal overlap of the two lasers, and second to normalize the fluorescence signal to power and frequency fluctuations of the UV laser. This procedure has the advantage of both being insensitive to UV pulse power fluctuations and being a factor of two faster than performing the experiment alternating with and without firing the IR laser. To perform this normalization the fluorescence was detected with a photomultiplier tube (Hamamatsu, R329-02) and integrated over two 0.25 us gates with two separate integrators (SRS, SRZSO) I One gate was set before and one after the IR pulse, which was fired z OS us after the UV pulse. The baselines of the integrators were carefully adjusted to yield zero volt output when the molecular beam was off. Both laser beams were paraliel to each other and crossed the molecular beam at right angles 80 nozzle diameters (0 = 0.3 mm) down stream from the nozzle. A computer controlled the data acquisition and also, by means of a programmable delay generator (SRS, DG 535), the timing of the experiment. The data analysis was performed by dividing the signals of the two integrators and normalizing the baseline of the signal. This yielded directly the fluorescence dip in percent.

3. Results and discussion In order to demonstrate IR spectroscopy in electronically excited states we have chosen propynal, the

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structure of which is given in Fig. 1. The pertinent spectroscopic features of the Si (A” ) +,(A’ ) transition of this near prolate asymmetric top molecule with C, symmetry are well knawn [ 12,13 1. Most rovibronic UV transitions are of C-type allowing us to resolve P-and R-branch transitions with our UV laser resohnion. Q-branch excitation involves, however, overlapping transitions. In the following we refer to the state pumped by the UV laser as the intermediate state and to the state accessed with the combined UVIR excitation as the upper state.

We first consider experiments, which were carried outwitha3cm-’ resolution of the IR laser sufficient to resolve vibronic transitions. The UV laser was tuned to the ‘Q. transition of the vibrational bands of interest, located in the energy range of 0 to 2600 cm-’ above the S, vibrational ground state, while the IR laser was scanned between 3250 and 3400 cm-‘. In all investigated bands, fluorescence dips were detected showing depths of up to 32% which decrease with increasing excess energy. Fig. 2 displays the depth of the dips at the lowest and the highest excess energy and Table 1 gives a survey of the examined vibconic states. Of particular interest is the dip observed after UV excitation into the 0: band and the IR frequency of 33 18 cm-‘. This frequency corresponds to the acetylenic C-H-stretching vibration v1 (see Fig. 1) in the S, state of propynal. Until now this has not been determined, because direct one-photon excitation of the tti vibration in the S, state from the vibrationless So state is F~nck-Condon forbidden For this experiment the IR laser was calibrated in a V-type IR-UV double resonance experiment using the frequency of v, = 3326 cm-’ in the So state as reported by Watson and co-workers [ 14,15 1. Some vibronic states exhibit more than a single dip indicating that the intermediate and/or the upper state is perturbed. The vibronic states may have different anharmonic coupling to the vibration excited by the IR laser, which often leads to widely separated dips. Furthermore, the dips show alternating intensities if different eigenstates of a ~~~urbatio~ are pumped with the UV laser. We illustrate this situation with the three vibronic bands shown in Fig. 3a,

Letters231 (1994)

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1.0

b)

i

0.8 i 3260

I

3300

7

3340

Idler I cm-1 Fig. 2. Fluorescence dips measured after pumping the ‘Qo branch of (a) the 0: band and (b) the 42 band in propynal with the UV laser and scanning the OPO with a 3 cm-’ bandwidth in the frequency range of the pi mode. The depth of the dips decreases with increasing SI excess energy.

which have been attributed to the Fermi resonance of the 48 UV band [ 13,15 1. They exhibit a separation of 11.6 and 2.4 cm-’ and are labelled (A), (B) and (C f . .After excitation of band (A) a single dip is observed within an energy range of igO cm-‘, whereas after pumping bands ( B ) and (C ) ) two dips separated by z 38 cm-i are detected for each band (see Table 1 and Figs. 3b-3d). This observation suggests that the interaction of the vibronic state accessed in band (A) with the vibronic states {B) and (C) is negligible. The separation of bands (B) and (C) restricts the maximum coupling matrix element to less than 1.2 cm-‘. Assuming a similar coupling strength between the upper states (B) 1’ and (C) I’, their large separation of 38 cm-’ implies that they are essentially unperturbed. Therefore, the involved transitions can be unravelled as depicted in Fig. 4 if only the interaction between the intermediate levels (B) and (C) is taken into account. The two strong transitions ( 1) and (2) in Figs. 3c and 3d are assigned to the zeroth-order transitions (B) -+ (B) 1’ and (C)G (C)l’, respectively, whereas the weaker transitions ( 3) and (4) occur because of state mix-

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Physics Letters 231 (1994) 6469

Table 1 Vibrational combination states of the acetylenic C-H-stretching vibration in the S, state of propynal measured with an IR resolution of 3 cm-‘. In the case several upper states arc given the state which was pumped by the UV laser shows a Fermi resonance according to Watson [ 15 ] Upper state

1’ 911’ 12’1’ 1021’ 6’1’ 5’1’ 4’1’(A) (B) (C) 4’9’1’(A) (B) (C) 4’12’1’(A) (B) 6’1’ 521’ 421’

S, excess energy 3318.0 3505.6 3646.2 4261.3 4275.0 4435.5 4614.4 4627.4,4591.0 4626.6,4590.2 4799.3 4816.6,4781.1 4781.4 4945.8 4951.4,4921.4 5201.2 5569.1 5909.6

A3”

x

.z 5 -c

Depth (96) 27456

0 - 1.4 -15.8 0.3 1.0 -0.5 -1.6 -0.6, -3.4, -0.9 -0.4, -39.6 - 14.4 - 18.6, -2.8 -0.9 - 1.4

-37.2 -39.8 -35.9

-48.6

a Ati gives the difference between the dip position monic value of the v, vibration in the S, state.

32 28 25 25 24 32 27 30, 13, 20 15, 15 20 20, 20 12 9

uv

15 24 10

). C

s

-E 15

and the har-

* 0

/

27463

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1

.o

(3)

c)

(1)

0.7

3260

ing in the intermediate level. Using the separation of 2.4 cm-’ for the states (B) and (C), a separation of - 36.4 cm-’ is obtained for the upper combination states (B) 1’ and (C) 1‘, where in contrast to the intermediate level the combination state (B) 1’ is higher in energy. From the transition energies and intensities a coupling matrix element of 1.15 * 0.0 1 cm-’ is calculated for the two interacting levels. Next we address the decrease of the dip intensity with increasing excess energy. The dip intensity depends on the IR transition matrix element, the fluorescence quantum yield of both the intermediate and the upper state, their respective lifetimes and the spectral response of the detection system to the emission spectra of the two states. In propynal the fluorescence suddenly breaks down at an S, excess energy of 4100 cm-’ [ 161. The vibrational state 1’ and its combination states with the low frequency modes vg and p12, 1’9’ and 1’ 12’, lie below this threshold (see Table 1) and their lifetimes are still sufficiently long to be measured by pumping first the vi vibration in the So state followed by the appropriate hot band

27474

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3280

3300

3320

3340

Idler / cm-’ Fig. 3. Fluorescence excitation spectrum of the 4: Fermi resonance (a) and the dip spectra observed for the three members of that resonance (b)-(d). In (a) the ‘Qc branches of the bands involved in the resonance are labeled with (A), (B) and (C). Fluorescence dips in (b)-(d) were measured after pumping the ‘Q,, branches of the three bands (A), (B), (C), respectively. For band (A) only one dip is observed, whereas in (B) and (C) two dips are found with alternating intensities (see text)

Fig. 4. Level scheme and transitions of the vibronic volved in the Fermi resonance shown in Fig. 3.

states in-

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electronic excitation [ 171. According to these measurements, these dips are the result of a reduced fluorescence quantum yield of the upper state compared to the intermediate state. However, for excess energies above the threshold the increased non-radiative rates shorten the lifetimes and reduce the quantum yields of the upper states so that the fluorescence is no longer detectable. In this case the fluorescence dips reflect directly the depopulation of the intermediate state by the IR laser and depend on transition moment and IR power. As all vibrational combinations were pumped with the same IR power, the weaker dip intensity implies a decrease of the transition matrix element. This effect may be explained by the following reasoning. Since in a normal mode picture separability of the vibrational modes would lead to a transition moment independent of the excited combination mode, the transition moment of v, must be distributed over several states by anharmonic coupling within the S, vibronic manifold or by interactions between electronic manifolds. Because excitation within the laser bandwidth of all the eigenstates created by such an interaction would recover the transition moment of the zeroth-order mode, the observed reduction of the transition moment indicates that only a fraction of the eigenstates has been excited within the 3 cm-’ IR bandwidth. Thus, the coupling must be considerably larger than the bandwidth of the IR laser.

Physics L&ten 231 (1994) 64-69

[ 161 can be studied. These three bands have relatively strong UV transitions and provide a good S/N ratio in the dip spectra. Favourable conditions were necessary, since the OPO operating at a bandwidth of 0.03 cm-’ generates IR radiation of considerably lower power than in the low-resolution mode. Fig. 5 shows some of the examined rovibronic transitions. Based on the rotationally resolved dip spectra of the 1; IR transition, the observed transition of the vi can be assigned as an A-type band. The width of the transitions is limited by the laser bandwidth, as in this band lifetimes of z 1 us were measured in direct IR-UV excitation experiments [ 171.

4=

3.2. Dynamics of rovibronic states Experiments in the frequency domain provide information on the intramolecular dynamics by means of linewidth, line shape and line splittings of the transitions. The homogeneous linewidth is related to the lifetime of the excited states and the coupling strength of a bound state to a continuum can be extracted from the line shape [ 18 1. Since for a reliable analysis spectral congestion has to be avoided, we investigated some of the upper states with an IR bandwidth of 0.03 cm-i allowing us to resolve neighbouring rovibronic transitions in propynal separated by 28x0.3 cm-‘. For these investigations single rovibronic transitions of the Og, 4h and 46 bands were selected with the UV laser. On adding a vi quantum by IR excitation, the dynamics of non-fluorescing states imbedded in the dissociation continuum HCCCHO+HCCCOH’ +H

-1

-0 5

0

0.5

Idler / cm-’ Fig. 5. Rotationally resolved fluorescence dip spectra after pumping the transitions (a) ‘&(J=2) of the 0: band, (b) s(J=3) of the 4:(A) band and (c) K(J= 1) of the 4; band and scanning the IR radiation with 0.03 cm-’ bandwidth in the frequency range ofthe or vibration. The rotational assignment of the IR transitions is given in the figure except for (c), where only the qR,(J=2) transition is shown. The solid line in (c) represents the tit of a Lorentzian with a width of 0.4 f 0.1 cm-’ (fwhh) to the data points.

Th. W&her et al. /Chemical

After increasing the excess energy by 1300 cm-’ in the (A) 1: band the rovibronic transitions are split into clumps of lines (see Fig. 5b). The clumps show a width of 0.4 to 0.6 cm-’ whereas the width of individual lines is determined by the laser bandwidth. Coherent excitation of such a clump of eigenstates would give rise to quantum beats and hence to dynamics on the picosecond time scale. After increasing the S, excess energy by an additional 1300 cm-‘, rovibronic transitions to the 421 ’ state are found to be broad and structureless as shown in Fig. 5c. For example the transitions to the J=3, K= 1 and the J= 4, K= 1 rotational states have linewidths of = 0.4 cm-‘, which correspond to lifetimes of about 30 ps if homogenous linewidths are assumed. In summary, we have demonstrated IR-absorption spectroscopy in conjunction with the fluorescence dip technique in an electronically excited state of a polyatomic molecule. This UV-IR double resonance method provides information on the spectroscopy and the dynamics of electronically excited molecules not only for the optically accessible, intermediate state, but most importantly for the upper state. In particular, rotationally resolved measurements of states with a lifetime even shorter than the time resolution of the detection system, make possible the investigation of the dynamics of states, which are not accessible by one-photon transitions or are non-fluorescing. With the advent of commercially available narrow-band OPOs, this method may soon become a convenient complementary method for studying structure and dynamic properties of molecular systems.

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Forschung, by the Werner Fonds and by the Ztircher Hochschulverein are gratefully acknowledged.

References

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Acknowledgement

[ 151 J.K.G. Watson, Ph.D. Thesis, Glasgow (1962). [ 161 P.R. Willmott, H. Bitto and J.R. Huber, Chem. Phys. 156

Support of this work by the Schweizerischer Nationalfonds zur Fijrderung der wissenschaftlichen

[ 171 Th. Walther, Ph.D. Thesis, Universitlt Zurich (1994). [ 181 U. Fano, Phys. Rev. 124 (1961) 1866.

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