Femtosecond multiphoton dynamics of higher-energy potentials

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CHEMICAL PHYSICS LETTERS

23 November 1990

Femtosecond multiphoton dynamics of higher-energy potentials R.M. Bowman, M. Dantus and A.H. Zewail Arthur Amos Noyes Laboratory ofChemical Physics I, California Institute of Technology, Pasadena,

G4 91125, USA

Received 30 August 1990; in final form 26 September 1990

The real-time motion of wave packets prepared coherently in different potential energy surfaces of molecular iodine is reported. Using multiphoton excitation and depletion techniques, we observe the phase-shifted oscillatory motion of the packet and the different transients characteristic of the bound B O+u( ‘H) and D O*u( ‘Z) state potentials alongwith the repulsive O+g( ‘I,) state potential. The approach helps in extending FTS to higher energy potentials and “dark” states, and illustrates experimental control schemes in a relatively simple system.

case (c)

1. Intmduction

selection rules for one-photon excitations

[151, In previous work from this group, femtosecond molecular dynamics on different potentials [ l-5 ]

have been probed using single-photon techniques. To access higher energy potentials, we extend the femtosecond transition-state (or temporal) spectroscopy, FTS, to the multiphoton domain. Several excitation and detection schemes are introduced, including fluorescence depletion. These techniques are demonstrated in experiments performed on molecular iodine. The results show the in-phase and outof-phase motion of a wave packet in the B 0% ( 3TI) state and transients characteristic of the bound D O+u(‘E) state and the repulsive O+g(rZ) state. Multiphoton spectroscopy of halogens (for a good review, see ref. [6] ) has been one of the most productive methods for investigating high-lying electronic states [ 7-131. The use of these methods allows the excitation of states otherwise unaccessible due to poor Franck-Condon factors caused by large shifts in the equilibrium internuclear distances. Onephoton-forbidden processes, for example g-g electronic transitions, can be probed via two-photon pathways. Highly excited ion-pair states of halogens, that require the generation of VUV pulses, can be reached with multiphoton excitation. Upon inspection of the excited state potential surfaces [ 141 of I2 and taking into account the Hund’s ’ Contribution No. 8211.

546

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0+-o-

,

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,

the following states can be accessed by one or more photons of 620 nm light starting in the X O’g(‘C) ground state: B O’u(9) (one photon), O+g( 5) (two photons), D O+u(‘C) -(three photons) and F O+u(‘C) (threephotons).TheB [16],D [7,17-221 and F [ 13,23,24] states are all bound and have been well studied by conventional spectroscopic methods, while little is known about O+g(‘G) [25], a repulsive electronic state. Both the D [ 111 and F [ 13 ] states have been studied by many multiphoton techniques including one-color, three-photon spectroscopy. It is believed that the dissociative O+g(‘2.) state enhances the three-photon transitions to these ionpair states [ 25 1. Thus, it seems that iodine is a good candidate for multiphoton FTS studies. The three possible cases that can arise from pumpprobe processes involving the aforementioned states are shown in fig. 1 and are listed below: pump :

probe :

X-B,

B-+O+g(‘C)+D, F,

(a)

X+B+O+g( ‘C) ,

O+g(‘Z)+D,

(b)

X+B+O+g( ‘E)+D , D-tO+g( ‘C) ,

(c)

0009-2614/90/$ 03.50 0 1990 - Elsevier Science Publishers B.V. (North-Holland)

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CHEMICAL PHYSICS LETTERS

-5olMlt

23 November I990

-5cm~

2.0 2.5 3.0 3.5 4.0 4.5 5.0

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2.0 2.5 3.0 3.5 41) 4.5 5.0

R (A>

R (A,

R (A,

11-photon/Z

-photon]

2-photon/

1-photon/

13-photon/depletion

1

Fig. 1. The potential energy surfaces (in cm-’ versus A) used in the multiphoton processesdescribed in the text. The X O+g(tZ), B O+u(‘H), D O+u( ‘C) and F O+u(‘E) state potentials are taken from the literature [6,16]. The dissociative O+g(‘Z) state iS approximated from refs. [ 14,251. The three schemes are appropriately labelled.

where each arrow corresponds to a single 620 nm photon transition. By the use of a one-, two- and three-photon pump process a wave packet can be prepared on the B O+u( 311) state correlating with I ( 2P312) and 1(*PI ,2); on the dissociative O+g( ‘C) which correlates with two I ( *PI,2) atoms; and on the D O+u( ‘1) ion-pair state which correlates to I+ ( 3P2) and I- ( ‘S ). The overall signal is followed by observing the fluorescence from either the D [ 7,17-221 or F [ 13,23,24] state, depending on the detection wavelength. Case (a) is similar to previous studies of iodine reported by this group (in this previous work one photon was used for probing) [ 5 ] ; the dynamics of a wave packet placed on the B state surface is followed. Case (b) corresponds to the dissociation of iodine on a repulsive surface. Note that only the D electronic surface can be reached as the final state in this process. In case (c), the D state is directly populated, and the wave packet motion can be followed by a one-photon depletion of the LIF signal from the D state. The present use of population depletion techniques and multiphoton methods extends FIS to

other domains of higher potentials and studies of “dark” states.

2. Experimental The experimental apparatus has been .described in detail elsewhere [ l-5 1. Briefly, the system consists of a CPM laser amplified by a 20 Hz Nd: YAG laser. These amplified pulses have an energy of up to 0.5 mJ and a pulse width (fwhm) as short as 50 fs at 620 nm. The only color necessary for multiphoton FTS is 620 nm, therefore the temporal resolution for all of the experiments was that of the red pulse, i.e. = 50 fs. The intensity of each beam was regulated with variably coated neutral density filters and each data point was normalized to the laser intensity. The relative polarization between the pump and probe lasers was kept parallel. The effects of different polarizations has been addressed previously [ 5 1, and will soon be the study of a detailed investigation. In the course of our investigation we have recorded transients for different pump and probe intensities and 547

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we have been able to observe cases ( a) and ( c ) , conclusively, while keeping all other experimental conditions constant. The intensities were changed over approximately a factor of 20. Using the highest intensities for pump and probe we obtained case (c) probing only (D state dynamics on both sides of the transients, see below), implying that the three-photon process was dominating. For lower intensities we could completely prevent the three-photon pathway. The designation of pump and probe beams is, of course, arbitrary and only depends on which process is being described. Low intensity laser pulses are more likely to result in one-photon absorption, while higher intensity pulses are going to discriminate towards multiphoton processes. In the remainder of the paper we will include the number of photons involved in each process along with the detection wavelength. For example, a one-photon pump to the B state followed by a two-photon probe to the D state, with the LIF signal detected at 4 10 nm, is designated 620 [ 1 ] / 620[2] (410). Thus, a one-photon promotion will imply a weak laser pulse was used and two- and threephoton promotions correspond to stronger laser pulses. It should be noted that the “negative time”, i.e. strong pump followed by a weak probe, signal occurs simultaneously, but on the opposite side of time zero as the 620[ 1]/620 [ 2 ] (4 10) process. This will be seen clearly in the following sections.

3. Results and discussion 3.1. Analysis of dispersed fluorescence spectra In order to understand the dynamics of the states accessed in our experiments, it is first necessary to assign the origins of the LIF signal. A dispersed fluorescence spectrum taken when the two femtosecond laser pulses are overlapped, i.e. time zero, is shown in fig. 2. In this case there is a weak pulse and a strong laser pulse, making it possible for all three proposed processes to occur. Several of the wavelength regions displayed in fig. 2 have been well studied and can be easily assigned. The region from 280 to 330 nm has been assigned to D O+u(‘Z)-tX O+g(‘C) fluorescence. This is easily seen by comparing this portion of the spectra to fig. 1 in ref. [21]. The 350 to 410 nm spectral region also originates from the D state 548

23 November 1990

5

2500

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4500

Wavelength(A) Fig. 2. The dispersed laser-induced fluorescence signal is shown where the pump and probe pulses are overlapped, i.e. time zero. The transitions responsible for each spectral region are shown. For the assignment of the fluorescence bands see text and refs. [ 17-19,21,24,26,27].

(compare to fig. 1 of ref. [ 19 ] ); the final state of this emission is believed to be a repulsive state, most likely the a’O+g state. The weak band at 270 nm is a well-known band due to the F O+u(‘E)-+X O+g( ‘C) fluorescence. This band is seen from a discharge of I*, as discussed by Tellinghuisen in fig. 1 of ref. [ 171, and is identical to the peak in our spectrum. These comparisons give us confidence in the assignments of the LIF dispersed spectrum. The only region unassigned is the peak around 340 nm. This area of the spectrum has been one of intense investigation and has lead to many erroneous assignments. One study of particular relevance to our work is that of Hemmati and Collins [ 181. In their work they excited ambient pressures of I, (with no buffer gases) at 275 and 300 K with a single excimer laser pulse at 193 nm (x 3500 cm-’ more energy than three 620 nm pulses). As can be clearly seen from their work (in fig. 2) [ 181, at 275 K there is no peak at 340 nm and, in fact, there is only a very small signal. When they heat their cell to 300 K (ten times higher IZ pressure) a prominent peak appears at z 340 nm. They attribute this to collisional electronic energy transfer from D O’u(‘C) to D’ 2g(311) with the subsequent emission of the well known D’ 2g(%+A’ 2u(%) band at 342 nm [24,26]. So even at these low pressures ( z 250 mTorr) a significant amount of collisional quenching can occur. Although it has not been as well documented, it has

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been postulated that the F state can also undergo electronic energy transfer to the D’ state [ 91. The peak at 340 nm is consistent with these findings. In the following sections all of the multiphoton processes we observed will be discussed, with careful attention paid to the effects of detection wavelength. In the process, we hope to demonstrate the utility of the multiphoton FTS approach.

cesses we observed by changing the intensities (of course, not all processes were observed with all intensity combinations). Therefore, if we can understand these representative transients, all of the multiphoton processes seen in our experiments will be explained. To observe the B state dynamics it is necessary to have a weak pump to preferentially excite this first electronic state. The left side of the transient, which corresponds to a weak pump-strong probe excitation scheme, should correspond to this situation. In fig. 4 (top) the transients are expanded and the time axis is reversed in order to make a clearer presentation of the data. In previous studies by this group using a one-photon probe [ 5 1, excitation at 620 nm is known to access vibrational levels 7- 12 of the B state, which corresponds to oscillations with a period of 300 fs. Immediate inspection of the left side of the transient, i.e. 620[1]/620[2] (410) and 620[1]/ 620[ 21 (340) experiments, reveals that both detection wavelengths display 300 fs oscillations. The 620[ 1]/620(2] (320) transient (not shown) is identical to the 620[ 1]/620[2] (410) as would be expected, since they both monitor D state LIE

3.2. Probing the B O+u(-ln)state dynamics Fig. 3 shows two experimental transients obtained with one weak 620 nm pulse and one stronger 620 nm pulse. The only difference between the two experiments is the detection wavelengths, all other experimental parameters were kept constant. As mentioned previously, it is important to view this transient as two experiments being performed simultaneously. Thus, the left side or “negative time” signal is monitoring one pump-probe process, while the right side or “positive time” follows another. The left side corresponds to a weak pump followed by a strong probe, while for the right side it is a strong pump followed by a weak probe. The different behaviors demonstrated in fig. 3 contain all of the pro-

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23 November 1990

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Time Delay (fs) Fig. 3. Femtosecond transients at two detection wavelengths, using weak pulse and strong pulseexcitations, are shown (see text).

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I-photon/depletion:

Time

D O+u(lZ:)

state]

Delay (fs)

Fig. 4. (top) The left side of fig. 3 is expanded with the time axis reversed. The 340nm data is arbitrarily raised to accentuate the differences in phase of the oscillations in the B O+u(%) state. (bottom) The right side of fig. 3 is shown. Again, the 340 nm data is raised. The oscilIations correspond to a wave packet moving in the D O+u( ‘C) state. Clearly the B state dynamics are followed using multiphoton probing. The most striking feature in fig. 4 (top) is the 180” phase difference in the oscillations between the 410 and 340 nm transients. Since the probe transition depends on the Franck-Condon overlap of the wave packet to the excited state, excitation to different higher states occurs at preferentially different internuclear distances. As shown schematically in fig. 1 for case (a), the probing process can take place at the inner and the outer turning points of the B state depending on the excited level accessed. In this case, the D state and F state favor the inner and outer turning points, respectively. Therefore, if the two detection wavelengths used here did correspond to the D and F state, we would observe signals that are 180” out of phase, as observed experimentally. This observation demonstrates the actual motion of the wave packet between the two turning points.

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The 620[ 1 ] /620[ 21 (410) is irrefutably D state detection, but the 620[ 1]/620[2] (340) is not obviously F state detection. Unfortunately, LIF detection at 270 nm, which is believed to originate from the F state, is too weak. The best evidence that the 620 [ I] /620 [ 21 340 data is coming from the F state is the peak (marked with an asterisk in fig. 3) that comes 150 fs after the first peak in the 620[ 1] / 620[2] (410) transient (180” out of phase). This peak corresponds to the pump pulse depositing a wave packet at the inner turning point of the B state, the packet then propagates to the outer turning point in 150 fs (half the period of oscillation) where it is two-photon excited to the F state (the FranckCondon-allowed state). The oscillations then continue at 300 fs intervals from this peak, consistent with F state LIF detection. Transient spectra obtained at times greater than zero (not shown) for the 620[ 1] /620[ 21 case display a large increase in the 270 and 340 nm fluorescence (see fig. 2), implying that both these regions of the spectra are monitoring the same state. As mentioned earlier, it is reasonable to assume that both D and F states are collisionally quenched to the D’ state, which is responsible for the 340 nm emission. There are a couple of possible reasons why the F state dominates the 340 nm fluorescence with this particular pump-probe scheme. It could be that the quenching of the F state is more eff!cient than the D state, thus even though both states channel into the 0’ state, the F state populates a larger fraction within the fluorescence lifetime of the D’ state. Another possibility is that the Franck-Condon factor is larger for the outer turning point as compared to the inner turning point for two 620 nm photons, resulting in a larger F state population as compared to the D state. The 620[ 1]/620[2] (340) experiment is therefore consistent with the F state description. The multiphoton excitation used in this case probes the dynamics of a wave packet placed on the B state electronic surface at both turning points. 3.3. Probing the D O+u ion-pair state dynamics: depletion experiments The right side of fig. 3 displays the results of experiments done with a strong pump and a weak probe, Oscillations with a period of 500 fs are ob-

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served for both detection wavelengths. This frequency is quite different from the one discussed in section 3.2 on the B state. A period of 500 fs corresponds to an average vibrational spacing of the wave packet prepared by the pump process of 67 cm - ‘, In the B state this would correspond to Y’= 32 and an excitation wavelength of E 530 nm, which is incompatible with our experiment. Simultaneous three-photon absorption can only access the D state (the F state cannot be reached by three-photon vertical excitation at 620 nm), which is the next bound state. Inspection of the D state potential parameters (of which there are several in the literature) offers the following. Using the parameters of three independent references [ 621,281, oscillations of 500 fs occur for V’= 124, 109 and 76, which correspond to a total excitation energy of 51050,49750 and 47280 cm-’ for refs. [ 6,21,28], respectively. These are all reasonably close to 48390 cm-‘, the excitation energy of three 620 nm photons coming from 0”=O (v” = 2, 3 are expected to have a contribution to the signal as discussed in ref. [ 51). Given the accuracy of these potentials for the D state, we feel the agreement is good enough to assign the 500 fs oscillations as originating from the D state. Three-photon excitation prepares a wave packet in the D state, which is then probed. The 410 nm detection is known to originate from the D state, thus we are directly detecting the D state dynamics by monitoring this fluorescence. This implies that the probe is depleting the fluorescence. Inspection of the potential energy surfaces [ 141 reveals .that the most likely process for depletion of the D state is a onephoton transition down to the dissociative O+g(‘E) state (or possibly two-photon depletion to the B state) at the inner turning point. Thus, the process can be described as 620[ 3]/620[ - l] (4101, where the [ - 1 ] means a downward transition in energy terms. A direct way of confirming the depletion experiment, shown in figs. 3 and 4, is the following. At time zero, the pump and depletion pulses are overlapped at the inner turning point of the D state potential. In 500 fs, the wave packet placed on the D state surface by the pump has undergone one full excursion, i.e. one vibration, and the wave packet has returned to the Franck-Condon region for the depletion pulse, Therefore, in 500 fs in the 620 [ 3]/620 [ - 1 ] case,

23 November

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a loss (or depletion) in the signal should be observed. This is clearly shown in fig. 4 (bottom). A dip in the signal is observed every 500 fs from time zero and the first peak in the transient is separated from time zero by = 750 fs. In contrast, the 620 [ 1 ] / 620( 2] (410) (in-phase oscillations, see fig. 4 (top)) experiment shows a peak every 300 fs with the first peak separated from time-zero by 300 fs. This is additional evidence that we are performing a fluorescence depletion experiment. Unlike the B state where the two detection wavelengths gave oscillations that were out of phase, detection at 340 nm produces an almost identical OScillation pattern. This is easy to understand. Since the only state that can be populated by the threephoton absorption is the D state, any collisional quenching to the D’ state which results in 340 nm emission must originate from the D state. Therefore, 620[3]/620[-I] (410) and 620[3]/620[-l] (340) both monitor the D state depletion dynamics. The femtosecond depletion experiments described in this section, like cw-stimulated emission pumping experiments [ 291 and picosecond depletion experb iments [30] by other groups, have proven quite powerful. We plan their further use in future FTS experiments. Finally within the first 200 fs, near time zero, we would expect to see evidence of case (b). The peaks near time-zero are partially due to this process as the actual “time-zero signal” is much more pronounced here as compared to the conventional one-photon pump-probe FTS work on I2 [ 5 1. Further experiments exploring the polarization and power dependences are planned.

4. Conclusions This Letter reports on the use of multiphoton FI’S techniques for higher-energy potentials. For molecular iodine, studied are two bound and one repulsive state. With a one-photon pump two-photon probe, the dynamics of the B state were recorded as we had done in earlier FTS experiments. With a three photon pump, a coherent wave packet can be prepared in the bound D ion-pair state, which was then probed by a fluorescence depletion technique. Additionally, the actual motion of the wave packet between the 551

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turning points can be observed as a phase shift in the

oscillatory transients. The phaseshifted oscillations of the packet are reminiscent of the work on IVR [ 3 1 ] and are consistent with theoretical predictions [ 321. The methodology promises to be very valuable in future experiments exploring higher energy potentials and “dark” state real t.ime dynamics. It also demonstrates the ability of controlling the wavepocket among different potentials in a relatively simple system.

Acknowledgement This work was supported by the AFOSR.

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