INTRODUCTION

TOF MS

3.1 INTRODUCTION

Studies of the dynamics of primary photofragmentation processes offer new op- portunities for the direct viewing of the process of bond breakage, its dependence on the nature of the transition state, and the resultant internal states of the prod- ucts. Information about (average) photodissociation lifetimes,

r,

has in the past been obtained from knowledge of the product angular distribution( s) and from the calculated average rotational period of the parent, w, as is illustrated, for the case of ICN, in the pioneering work of Ling and Wilson.¹ To obtain

r,

however, one must assume a model for the sharpness of the angular distribution and rely on the classical model for relating

r

and w to the spatial anisotropy,

/3,

of the reaction.

Picosecond and femtosecond photolysis-and-probe techniques allow one to obtain r directly and test the validity of the models involved in the description of the rota- tion of the parent and the recoil process. Comparison of the direct time-resolved results with steady-state angular distribution measurements can then be made.

In recent work from this laboratory we have reported on the picosecond photolysis-and-probe monitoring of chemical reactions in molecular beams.²- 4 The primary process of alkyl iodide, RI, bond fragmentation to R and I was not, how- ever, resolved because the fragmentation, in this case, is on a repulsive surface and the time scale for dissociation is shorter than the picosecond resolution of our

'•"

experiment.

In this Letter we wish to report our first results on the femtosecond time- resolution of the primary photofragmentation ( on the repulsive surface) of the re- action

I-CN - [I· · -CN] - I

+

CN

In these experiments, as outlined in Figure 1, a femtosecond pulse (photolysis pulse) initiates the dissociation by exciting ICN (with a well defined E-field polarization direction in the laboratory frame) to the continuum absorption of the repulsive

A

state with 306-nm light. The second pulse (probe pulse) monitors the recoiled CN at 388 nm ( with its E-field orientated parallel or perpendicular to that of the photolysis pulse polarization). By observing the laser-induced-fluorescence of the rotationally excited CN fragment as a function of the time delay between the pho- tolysis and probe pulses we measure the build-up (rise) time for the formation of the CN photoproduct. These experiments provide a direct view of the process of bond breakage and illustrate some of the difficulties inherent in the earlier indirect methods.

3.2 EXPERIMENTAL SECTION

3.2.1 Apparatus

The two femtosecond pulses ( 306 and 388 nm) were generated by using the fol- lowing arrangement: A mode-locked argon ion laser synchronously pumps a cavity dumped dye laser (pulse width 5ps, 612nm), the output of which is compressed in a fiber optic/ grating double-pass arrangement. The compressed pulse autocorrelation is typically 350-400 fs in duration. These pulses are amplified in a three stage dye amplifier which is pumped by the second harmonic of a Nd: YAG laser. The first two gain stages are isolated from each other by a spatial filter, while the second two are optically separated by a dye jet of the saturable absorber DQOCI. The resultant _,.,, amplified pulses have typical energies of 150µJ and are usually 400fs in duration.

The pulse compressor is adjusted to precompensate for the dispersion encountered in the amplifier which, along with the saturable absorber jet, serves to prevent any significant pulse broadening. Mixing the amplified pulse in a nonlinear crystal with the l.06µm YAG fundamental produces the 388-nm probe light, while the 306-nm light is the second harmonic of the amplified fundamental. This method of light generation produces pump-and-probe pulses with an insignificant amount of relative timing jitter. We have also taken care to ensure that we are able to phase-match the entire frequency bandwidth to minimize the extent of pulse broadening in the two mixing processes.

The 306- and 388-nm pulses traverse seperate arms of a Michelson intrferom- eter, one beam path containing a fine-resolution variable delay. The relative po- larization is adjusted with a Soleil-Babinet compensator /thin film polarizer com- bination in the 388-nm arm. The beams are recombined and adjusted to travel collinearly through the experimental (ICN) and response function cells. The ICN cell is extensively baffled to reject scattered laser light and allow straightforward detection of the resonance laser induced fluorescence from the CN fragment. The response function of the system is obtained by replacing the ICN-LIF cell with an ionization cell of N ,N-diethylaniline (DEA) and generating a resonance enhanced 1+1 ionization transient. The cell repositioning is done without changing any of the overlap adjustments or adding neutral density filters, therefore, a calibrated set of data (ICN transient, DEA REMPI response) is obtained. Calibration of the ICN transient against the REMPI response allows us to measure the rise and the "t=0 shift" for the 306/388 nm pulse excitation instead of relying on the autocorrelation of the dye laser pulses. Finally, the handling of ICN and the signal processing are straight forward and have been described elsewhere.⁴

3.2.2 Treatment of the Data

Figure 2 displays the apparent rise and shift observed in these femtosecond photoysis-and-probe experiments. This form of the delayed rise is similar to the observation made by Smith et al.⁵ on the picosecond time scale. To obtain r

from our measurements we have treated the data in the following way. We used the REMPI transient as the response function of the system for fitting the ICN transient using a non-linear least squares single exponential fitting routine. This gave r

=

600

±

^{100 fs.}

The REMPI transient rises and becomes flat with no indication of any de- cay components. This indicates that the resonant intermediate is long lived ( ns) and that the observed REMPI is the response function for the 306/388 nm experi- ment. We have also considered the possibility that the intermediate state of DEA has an additional fast decay component due to off-resonant ionization and/or IVR processes. ⁶ In this case a biexponential decay is the expected behavior and the fast component can result in further sharpening of the response function, leading to an apparent shift from the actual system response. We have modeled the observed rise by convoluting a gaussian pulse ( the integration of which gives the ICN transient) with a decaying biexponential function, fixing the long component at 8 ns decay and the fast component as pulse width limited ( to be of the order of the pulse). It is found that the experimentally observed shift cannot be obtained while simultane- ously maintaining the shape of the REMPI transient. When the experimental shift is matched the modelled REMPI response shows a pronounced decay component, which is inconsistent with the experimental observation. We found that by making the fast component much shorter (

<

10 fs) we could, in principle, reproduce the shift but with the ratio of the fast to slow component being unphysically large (

> 50).

The above treatment of the data confirms that the REMPI transient is the system response and that the observed shift is due to the finite r of 600

±

lO0fs.

This is consistent with several experimental facts: (1) We observe similar temporal behavior when the polarization of the probe is perpendicular to that of the photol- ysis pulse. (2) The temporal behavior is not sensitive over a range of pulse energies, and the ion signal is linear in the probe intensity. (3) The pump wavelength is on

resonance with the absorption of DEA, which at room temperature is continuous at 306 nm.⁷

Finally, to reiterate, the fitting of the ICN rise (600

±

lOOfs) was a convolution of the measured response function with a single exponential build-up, reflecting the prompt photodissociation process on the repulsive surface( s ).