J. Phys. Chem. 1995, 99, 3881-3882
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COMMENTS Comment on “Femtosecond vs Nanosecond Multiphoton Ionization and Dissociation of Large Molecules” Richard Knochenrnuss LOC,Universitatstrasse 16, ETH-Zentrum, 8092 Zurich, Switzerland Received: October 20, 1994; In Final Form: January 11, 1995
In the title study,’ Weinkauf, Aicher, Wesley, Grotemeyer, and Schlag (WAWGS) demostrated that, for large molecules, there can be significant differences in multiphoton ionization yield and patterns of molecular fragmentation resulting from excitation with sub-picosecond vs nanosecond laser pulses (with similar photon numbers). For multiphoton ionization, shorter pulses were found to be as much as 50 times more efficient at producing ions from molecules of mass > 1000 Da (gramicidin S, 1141 Da, and gramicidin D, 1881 Da). For multiphoton ionization and fragmentation, shorter pulses result in bond breakage near the chromophore of large molecules, while longer pulses yield fragments due to bond breakage throughout the molecule. Both of these results were interpreted in terms of the relative rate of vibrational energy flow in the molecule vs the rate of optical pumping. Slow pumping allows excess energy (above the absorption band origin) to be randomized in the molecule between multiple absorptions, while fast pumping creates ions and fragments before energy can spread throughout the molecule. WAWGS obtained these interesting results with 5 ns and 500 fs dye lasers. The nanosecond system was constructed and operated so as to minimize possible temporal fluctuations in the output pulses. This Comment is intended to add utility and perspective to those results by presenting temporal profiles of pulses from nanosecond lasers more typical of those in use in many laboratories. Figure 1 presents streak camera traces of single shots from a Lambda Physik FL-3002 dye laser in the factory standard configuration (without etalon), using Rhodamine 110 dye, and tuned for good line width and stability, Le. in normal use. Both fundamental 564 nm and doubled 282 nm pulses are presented. A small fraction of the output was split off with a quartz flat, attenuated with metal film neutral density filters, and focused onto the entrance slit of a Hamamatsu streak camera. This has a single microchannel plate image intensifier and a silicon diode array detector. Nominal time resolution was 14 ps. The full pulse widths were about 15 ns, and the leading edges of the pulses are shown. The laser was pumped with 100-120 mTl pulse from a XeCl excimer laser at 308 nm. As can be seen in the figure, the output intensity of this dye laser was strongly modulated in time. The modulation is not reproducible, but there are general similarities between shots. The spikes of 100-300 ps full width are presumably a result of mode beating in the oscillator, since this type of moderate resolution laser is not designed to operate in a single mode.
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Figure 1. Streak camera traces of the fundamental (564 nm) and doubled (282 nm) outputs of a Lambda Physik FL-3002 dye laser, pumped by a XeCl excimer laser at 308 nm. Each trace is the leading edge of a single laser shot.
The depth of modulation is, of course, increased after the nonlinear doubling process. Detailed studies of the parameters affecting the temporal structure were not carried out; minor tuning adjustments were not found to have strong effects, nor was the age of the dye solution. It should also be noted that the approximately 25 ns pump pulses had a smooth, although not Gaussian, envelope. The observed modulation is thus entirely due to the dye laser. The full beam did not enter the narrow slits needed for high time resolution; the observed fluctuations would be reduced somewhat if the full beam were averaged. On the other hand, using an unseeded Nd:YAG pump laser will induce further temporal structure in the dye laser output, since such lasers also show spiking, with typical widths of a few hundred picoseconds. WAWGS used high pump energies (190 dlpulse) and a separate, optimized, saturated amplifier rather than the one which is standard in the FL-3002dye laser. Such differences vs the system used here should have resulted in a significant reduction of the amplitudes of the output spikes. On the other hand, they used a nonseeded pump laser, which could slightly increase the spiking. In any case, the conclusions of their study are not affected, since even spikes such as those presented here are 3 orders of magnitude longer than their short pulses, so a good separation of fast and slow time scales remains. Any spiking in their nanosecond pulses would also tend to reduce rather than increase the time scale effect they found. The data presented here show that users of standard nanosecond dye lasers should know that, under some conditions, they may actually be using a train of sub-nanosecond pulses. This could influence interpretation of some experiments,
0022-3654/95/2099-388 1$09.00/0 0 1995 American Chemical Society
3882 J. Phys. Chem., Vol. 99, No. 11, 1995 especially if one were to compare results with excitation in the sub-nanosecond rather than sub-picosecond range. Further systematic exploration of the temporal pulse structure of common laser systems would seem to be of value to the research community.
Acknowledgment. The author thanks Dr. R. Weinkauf for
Comments further information on the WAWGS experiments and for valuable comments.
References and Notes (1) Weinkauf, R.; Aicher, P.; Wesley, G.;Grotemeyer, J.; Schlag, E. W. J. Phys. Chem. 1994, 98,8381
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