178
J . Phys. Chem. 1984, 88, 178-181
Studies of Rapid Dynamics in Laser-Multiphoton Ionization Dissociation Mass Spectrometry by Using Pump-Pump Two-Color Picosecond Lasers: 2,4-Hexadiyne D. A. Gobeli, Jack R. Morgan, R. J. St. Pierre, and M. A. El-Sayed* Department and Chemistry and Biochemistry, University of California. Los Angeles, California 90024 (Received: October 3, 1983)
The quasiequilibrium theory of mass spectrometry assumes that ionic dissociation occurs after relaxation to a statistical distribution of vibrational energy levels in the ground electronic state of the parent ion has taken place. This Letter presents the results of an attempt to intercept and determine the rate of these relaxation processes prior to laser multiphoton ioni2ation dissociation. Observed changes in the mass spectrum of 2,4-hexadiyne as a function of the delay time between two picmecond laser pulses of different wavelengths suggest that the relaxation processes are on the same time scale as the delay times used. The results are discussed in terms of rapid radiationless processes in the lowest singlet state of the parent molecule, the previously observed lifetime of the first electronic excited state (A ’E,) of the parent ion, and the statistical behavior of the ionic dissociation process to give C4H4+.
Introduction Quasiequilibrium theory’ assumes that ionic dissociation is much slower than the relaxation processes leading to the statistical distribution of the internal energy among the vibrational degrees of freedom of the ground electronic state of the parent ion. If the dissociation times are on the microsecond time scale, i.e., comparable to the ion extraction time, metastable peaks are observed in both and multiphoton mass ~ p e c t r a . ~ From analyses of these peaks, dissociation times are determined. If dissociation takes place in the field free flight region (longer than a few microseconds) in a time-of-flight mass spectrometer, reflectron techniques4 can be used to determine dissociation times. The presence of rapid relaxation processes prior to dissociation which is proposed in the quasiequilibrium theory is usually supported if the satistical calculations give breakdown curves that are in agreement with experiment. Observation of statistical scrambling of isotopically substituted compounds also supports the statistical t h e ~ r y . Unfortunately, ~ there is no direct experimental method present that can intercept and determine the relaxation times of the processes preceeding dissociation in mass spectrometric studies. The present Letter is an attempt in this direction. Recently, studies of the mechanisms of laser multiphoton ionization dissociation in mass spectrometry have become very active with the use of one or two lasersb” or electron impact with laser~.l’*’~Lasers as an ionization source possess some advantages and disadvantages over more conventional forms of ionization. Because of the short pulse duration of lasers (pico- and subpicosecond), the MPI has potential advantage in time-resolved studies of molecular and ionic dynamics. This potential is explored and demonstrated in this Letter. We developed a technique that is based upon the following simple idea. Two lasers of short pulse duration and variable delay (1) Rosenstock, H. M. Adv. Mass Spectrom. 1968, 4, 523. (2) Hipple, J. A. Phys. Rev. 1947, 71, 594. (3) Proch, D.; Rider, D. M.; Zare, R. N.; Chem. Phys. Lett. 1981,81,430. (4) Boesl. U.: Neusser, H. J.; Weinkauf, R.; Schlag, E. W. J . Phys. Chem. 1982,86, 4857. ( 5 ) Levsen, K. “Fundamental Aspects of Organic Mass Spectrometry”; Verlag Chemie: New York, 1978; Vol. 4. (6) Antonov, V. S.;Letokhov, V. S.Appl. Phys. 1981, 24 89. (7) Boesl, U.; Neusser, H. J.; Schlag, E. W. Z. Naturforsch. A 1978, 33, 1546. (8) Antonov, V. S.;Knyazev, I. N.; Letokhov, V. S.; Matiuk, V. M.; Movshev, V. G.; Potapov, V. K. Opt. Lett. 1978, 3, 37. (9) Reilly, J. P.; Kompa, K. L. J . Chem. Phys. 1980, 73, 5468. (10) Zandee, L.; Bernstein, R. B. J. Chem. Phys. 1979, 70, 2574. (1 1) Pandolfi, R. S.;Gobeli, D. A.; El-Sayed, M. A. J. Phys. Chem. 1981, 85, 1779. (12) Dunbar, R.; Armentrout, P. Znt. J. Mass Spectrom. Zon Phys. 1977, 24, 465. (13) Newton, K.; Bernstein, R. B. J . Phys. Chem. 1983, 87, 2246.
0022-3654/84/2088-0178$01.50/0
times with respect to one another are used to produce the MPID mass spectra of a molecule under collision-free conditions. If the peak intensity of a certain fragment ion changes with the change in the delay time, then processes leading to its formation must be occurring on the time scale of the delay time used. The time resolution of this method is limited by the laser pulse width. The molecule studied is 2,4-hexadiyne for which important spectroscopic information is known for both the neutral14J5 and its as well as the results of photoion-photoelectron coincidence e~periments.~~J* The Quantel YAG (nano-pico) laser is used. The fourth harmonic (at 266 nm) is used as the primary pulse, and the second harmonic (at 532 nm) is used as the secondary pulse. From the energy level diagram and changes in the intensities of the parent ion and C4H4+peaks as a function of the delay time, results concerning the picosecond and nanosecond dynamics in the molecule and its parent ion prior to dissociation are obtained These results are discussed in terms of the statistical theory. The observed short lifetime of the lowest singlet states of 2,4-hexadiyne suggests its potential use in pulse width determination of high-energy picosecond lasers with MPI autocorrelation technique^.^^^^^ Experimental Section The second (532 nm) and fourth (266 nm) harmonics of a Quantel Nd:YAG laser provided laser pulses of 25-ps duration. The two pulses were separated by means of a Pellin-Broca prism and then recombined with a harmonic beamsplitter after the second harmonic traversed a variable optical delay line of 0 to 10 ns. With a 150-mm focal length quartz lens, the recombined beams were then focussed between the first two electrodes of a 2-m, differentially pumped, home-built, time-of-flight mass spectrometer. The 2,4-hexadiyne was obtained from commercial sources and used without further purification after analysis by G C / M S revealed negligible impurities. Sample pressures within the ionization region of the mass spectrometer were kept at approximately 1 x 1 0 - ~torr. Ions generated in the MPI process were detected by a Channeltron electron multiplier (Galileo Electrooptics). The signal was amplified with a video amplifier, recorded on a fast transient (14) Price, W. C.; Walsh, A. D. Trans. Faraday SOC.1945, 41, 381. (15) Woo, S.:Chu, T. C.; J . Chem. Phys. 1937, 5, 786. (16) Allan, M.; Maier, J. P.; Marthaler, 0.; Kloster-Jensen, E. Chem. Phys. 1978, 29, 331. (17) Danacher, J. Chem. Phys. 1978, 29, 339. (18) Baer, T.; Willett, G. D.; Smith, D.; Phillips, J. S.J. Chem. Phys. 1979, 70, 4076. (19) Morita, N.; Yajima, T. Appl. Phys. B 1982, 28, 25. (20) Rayner, D. M.; Hackett, P. A.; Willis, C. Rev. Sci. Znstrum. 1982, 53, 537.
0 1984 American Chemical Societv
The Journal of Physical Chemistry, Vol. 88, No. 2, 1984 179
Letters
4500]
C,H,'
ION CURRENT
c
J
I
1
n
15.1 15.2 14.5
--
.14.60 14.05 13.55
-- J I W
2000 -100
14.3
-50
50
DELAY
TIME
100
U
(ps)
Figure 1. The effect of the delay between the UV (266 nm) laser pulse (25 ps) and the green (532 nm) laser pulse on the 2,4-hexadiyne cation mass signal. At zero delay, the intensity of the parent ion almost doubles, suggesting a nonradiative process in SI of the neutral molecule formed by one-photon UV excitation. The shape of this peak is very similar to the 532-nm autocorrelation peak obtained by using KDP nonlinear crystals (determined by the laser pulse width of 25 ps). This suggests that the lifetime of S1 in the neutral molecule is
