Femtosecond interferometric photoacoustic spectroscopy using incoherent light

16 August 1996

CHEMICAL PHYSICS LETTERS

ELSEVIER

Chemical Physics Letters 258 (1996) 460-464

Femtosecond interferometric photoacoustic spectroscopy using incoherent light L . C . S n o e k a, S . G . C l e m e n t a, E J . M . H a r r e n b, W.J. v a n d e r Z a n d e a,h a FOM Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands b Department of Physics, University ofNijmegen, Toernooiveld, 6525 ED Nijmegen, The Netherlands

Received 29 April 1996

Abstract

An interferometric phase-modulated femtosecond wave packet technique has been combined with photoacoustic detection to study rovibrational wave packets, using 30 ps (full-width-at-half-maximum) incoherent light pulses. The phase-modulated technique is particularly suited to be combined with photoacoustic detection, thanks to the adaptability of the modulation frequency of the light pulses to correspond to a resonance frequency of the photoacoustic cell. As an example to demonstrate the principle, the B-state of iodine has been chosen. The average 330 fs wavepacket period of the vibrational levels 12-15 and wave packet dephasing have been observed, identical to those which would be expected if transform-limited, 100 fs fwhm pulses had been used. It is shown that the combination of these detection techniques results into a very sensitive method for performing time-resolved experiments.

1. Introduction

Recent developements in laser technology have allowed for real-time femtosecond pump-probe studies of gas phase compounds using a variety of techniques. There are essentially two types of two-pulse pumpprobe experiments. The first one, two colour pumpprobe spectroscopy involving femtosecond pulses, has been used to observe vibrational wave packet motion in a number of diatomic molecular systems [ I - 3 ] . These experiments can yield spatial information about the evolution of the vibrational wave packet, if the probe excitation cross section is appreciably dependent on the internuclear distance. Another set of experiments, based upon Ramsey's separated oscillatory fields method [4,5], involves excitation by two identical optical pulses separated by a time delay ~-, without noticeably depopulating the ground state. As ~- is

varied, the excited state population exhibits fast oscillations at the average excitation frequency with a slow modulation at a frequency corresponding to approximately the average energy level spacing between the excited states. As has been pointed out by Lankhuijzen et al. [6], spectra taken with the optical Ramsey method yield the exact Fourier transform of the convolution of the frequency spectrum with the spectrum of a short optical pulse. In the group of Fleming, phase-locked femtosecond pulse pairs were used to obtain rovibrational wave packet interferograms of the B-state of molecular iodine [7,8]. A variant on this technique has been developed in our laboratory by Noordam and Broers [9-11 ] : instead of actively locking the relative phase of the two pulses, phase modulation was used to eliminate the need for an interferometrically stable delay line. This method has been succesfully applied to, e.g., the study of Rydberg elec-

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L.C. Snoek et al./Chemical Physics Letters 258 (1996) 460-464

tron wave packets and low-n state fine-structure wave packets in atomic rubidium [ 12,13]. In the group of Girard [ 14], the need for active phase locking of the pulse pairs was circumvented by using an extremely stable optical table and delay line. Recently, Jones et al. [15] and Kinrot et al. [16] have shown that it is not necessary to know the phase relationship between the pulses to yield interferograms. More interestingly, Jones showed that the resulting interferograms are only sensitive to the excitation cross section and the pulse frequency spectrum, and not to the pulse duration. The results of such experiments are similar to those obtained using the two colour pump-probe technique developed in the group of Zewail, but in contrast, give no spatial information about the wave packet and yield only the time-autocorrelation function. Therefore non-transform-limited pulses of the same bandwidth (but of a much longer pulse duration) could have been applied to obtain the same results as with femtosecond transform-limited pulses. In this Letter we aim to illustrate the practical use of a photoacoustic cell in optical Ramsey interferometric experiments. The modulation technique is particularly applicable to be combined with photoacoustic detection, because of the adaptability of the modulation frequency of the light pulses to be matched with a resonance frequency of the photoacoustic cell. As an example this technique has been applied to the study of rovibrational wave packets in the B-state of molecular iodine. This specific molecule has been taken both for experimental convenience, its well known B*--X transition is in the visible region of the spectrum, and because it is a well documented system (see e.g. Refs. [ 17,18] ). In the experiment 30 ps fwhm incoherent pulses were used, which proves again that only the coherence length of the two pulses is important, not the pulse duration.

2. Experiment Photoacoustic spectroscopy is based on the fact that the absorption of light by a medium heats it up (unless the energy is reemitted as light). If the light intensity is modulated, the temperature of the medium will fluctuate. For a gaseous absorber, the temperature fluctuations will cause pressure variations (sound waves), which can be detected by a microphone if the modu-

461

lation is performed at an audio frequency. Very high sensitivity, as required for e.g. overtone spectroscopy or trace gas detection, can be achieved by designing the absorption cell as a resonant cavity for the modulation frequency [ 19,20]. In most standard photoacoustic experiments the light used for the excitation is modulated by chopping the (cw) laserbeam. In our case, the two-pulse phase-sensitive interference technique [ 10] was applied to modulate the phase of the incident light at a resonance frequency of the cell. The output of a free running, cavity-dumped, 3.8 MHz rep. rate picosecond dye laser (Coherent 700, R6G dye) was used to obtain the large bandwidth that was required to coherently excite several vibrational levels. It was synchronously pumped by a frequency-doubled modelocked Nd:YAG laser (Coherent Antares, 75 MHz rep. rate, 100 ps pulses). By removing the birefringent filter, non-bandwidth-limited, broadband ( ~ 300 cm- i ) pulses were obtained, of which the phase autocorrelation width as measured in the experiment was about 120 fs. Outside of the dye laser an interference filter of 580 nm central wavelength and a bandwidth of 9 nm fwhm (Oriel 53910), could be used to obtain a better defined wavelength (as required for the simulations), since the output frequency of the dye laser was changing almost every shot. The intensity autocorrelation width of the dye laser pulses was ..~ 33 ps. The wavelength of the pulses was centered around 577 nm, which corresponds to the excitation of the rovibrational levels v' = 12-15 of I2(B), as well as a minor contribution of v' = 14-17 and v' = 16-19, which arise from hotband transitions originating from ground state levels v = 1,2 respectively. Fig. 1 shows a schematic view of the experimental set-up. In a Michelson arrangement each laser pulse was split into two identical ones, which could be time delayed relative to each other. In the fixed arm of the Michelson, a wiggling quartz plate was used to modulate the relative phase of the pulse pair at a frequency of approximately 1 kHz, one of the resonance modes of the cell. The PAC contains an open end type resonator of 300 mm length and 6 mm diameter, and is filled with a mixture of I2/N2 at room temperature and standard pressure. Four Knowles electret microphones (EK3033) are positioned on the central perimeter of the resonator. The resonator is excited in its third longitudinal mode to obtain a maximised signal-to-noise ratio. The acoustic signal is amplified

L, C. Snoek et al. / Chemical Physics Letters 258 (1996) 460-464

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and sent through a 1 kHz band pass filter, the frequency of which matches the modulation frequency of the phase between the two pulses. The intensity variation of the wave packet signal as a function of delay time is obtained after squaring the output of the filter and averaging it. During the test measurements it appeared that the photoacoustic set up was very sensitive to external acoustic noise. Therefore the whole cell was confined in a padded steel box, which dramatically improved the signal-to-noise ratio.

3. Results and discussion In Fig. 2 we present the femtosecond interferometric signal as a function of the delay between the pump and probe pulses. The spectrum has been normalised using the signal at 7" = 0, since there the time autocorrelation function I ( 0 ( 0 ) I 0 ( z ) )l ~ I. The zero intensity level has been chosen to be at long delays where the signal has completely dephased, the top of the zero-delay signal has been defined as the maximum intensity level (unity). Although the signal at later delays is only on the order of a few percent of the zero-delay signal, the very good signal-to-noise ratio allows to observe here an oscillation period of 330 fs. This period corresponds to the average vibrational frequency of 12 (B, v p =12-15) that is expected for 577 nm excitation. This 330 fs periodic structure

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Delay (ps) Fig. 2. Photoacousticexperimental recurrence spectrum of 12, taken without the interference filter: note the 330 fs period, corresponding to the average vibrational level spacing of the 12(B. t,r = 12-15) levels of 102 cm-l, and the 1 ps period, an artefact of the optical Ramseymethod, resulting from energy differences between different hotbands. is due to the interference between the wave packets created by the first and second pulse and is a measure for the time autocorrelation function. Thus, each recurrence indicates that the wave packet has returned to something like its initial configuration. Then, after approximately 8-10 vibrational periods, the signal decays due to dephasing of the wave packet. A second, slower oscillation modulates the fast 330 fs recurrent signal at ~ 1.6 ps. This is caused by the presence of hotbands in the absorption spectrum, the energy differences of which correspond to features in the time domain between 0.75 and 2.0 ps. One has to realise that this specific 1.6 ps period can never be observed in a true (two-colour) pump-probe experiment, since energy differences originating from different hotband excitations will never give rise to recurrences in the time domain. The fast decay of the signal shortly after r = 0 is caused by the excitation of the multitude of ground state rotational levels populated at T = 300 K. The experiment was repeated using the narrow band interference filter, which provided a well defined central frequency of the light used for excitation. This enabled us to simulate the recurrence spectra, implementing the known experimental conditions. In Fig. 3 the simulation procedure is presented. In Fig. 3A a simulation of the 300 K 12 energy spectrum is shown (thin line), which has then been convoluted with the

L.C Snoek et al./Chemical Physics Letters 258 (1996) 460-464

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spectrum of a Gaussian 100 fs pulse centered at a wavelength of 577 nm (bold line). This convoluted spectrum is Fourier transformed to yield the simulated interferogram shown in Fig. 3B (bold lines). The agreement between the simulated recurrence spectrum and the experimental data is good, but at longer delays (7" > 5 ps) the experimental signal is reduced to less than 0.1% of the zero-delay peak, which thwarts the observation of the modulations predicted by the simulation.

Acknowledgement The authors would like to thank Albert Martis of the Katholieke Universiteit Nijmegen for his assistance in the first operation of the photoacoustic cell. This work is part of the research programme of the Stichting voor Fundamenteel Onderzoek der Materie (FOM), and was made possible by financial support from the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO).

References 4. Conclusion We have shown that the technique of phasemodulated wave packet interferometry can be applied using non-transform-limited pulses, the bandwidth of which covers the portion of the excitation spectrum of interest. The modulation technique is particularly suited to be combined with photoacoustic detection, thanks to the adaptability of the modulation frequency of the light pulses to match a resonance frequency of the photoacoustic cell. Consequently, this combination results into a very sensitive detection technique for time-resolved experiments.

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