J. Phys. Chem. 1981, 85,15-17
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Two-Color Laser Study of the Multiphoton Ionization and Fragmentation of Triethylenediamine K. R. Newton, D. A. Llchtln, and R. B. Bernstein” Chemlstty Department, Columbia Unlverslty, New York, New York 10027 (Recelved: October 24, 1980)
A two-color laser study of the resonance-enhanced multiphoton ionization and fragmentation of gaseous triethylenediamine is reported. When an initial pulse (at 559 nm) is used to prepare the A state by a two-photon excitation, a second pulse (355 nm), delayed appropriately,excites the residual A state molecules by a one-photon process directly to the continuum to yield parent ions in abundance. Without this second pulse, parent ion formation is negligible: the A state is efficiently pumped (by 559-nm photons) “up the ladder”,yielding mainly small ion fragments. The experiments elucidate pathways and energetics of the fragmentation process.
I. Introduction Recently,l-l0there has been considerable interest in mass spectrometric determination of the ion fragments resulting from resonantly enhanced multiphoton ionization (REMPI) of isolated molecules. Using multicolor excitation, one can probe some of the finer details of the REMPI process. This Letter reports results of a two-color REMPI experiment on the molecule N(C2H4)3N,triethylenediamine (Dabco), which amplify previous two-color REMPI results obtained by Parker and El-Sayedl’ in a gas-cell ionization chamber. Recent experiments on Dabco carried out in this 1aboratorylO with a computer-controlled laser ionization time-of-flight (TOF) mass spectrometer showed that different fragmentation patterns were obtained depending upon the excitation wavelength (corresponding to different intermediate states). Presented here are results of twocolor experiments in which TOF mass analysis is carried out on the ions resulting from an initial laser pulse at one characteristic excitation wavelength in the green (559 nm) followed by a delayed pulse of UV light (355 nm). These mass spectral data serve to clarify the mechanism of the REMPI fragmentation process for Dabco. 11. Apparatus and Procedure The apparatus consisted of a Quanta-Ray Nd:YAG (DCR-1A) pumped flowing dye laser (PDL-l), a CVC MA-2 time-of-flight mass spectrometer (TOFMS), a Biomation 6100 waveform recorder and a D.E.C. MINC-11 microcomputer. The system is schematically portrayed in Figure 1. The main features of the instrument have been described previously.1° The only significant changes introduced here are in the laser system and optics. In the (1) L. Zandee, R. B. Bernstein, and D. A. Lichtin, J. Chem. Phys., 69, 3427 (1978). (2) V. S. Antonov, I. N. Knyazev, V. S. Letokhov, V. M. Matiuk, V. G.Movshev, and V. F. Potapov, Opt. Lett., 3, 37 (1978). (3)U. Boesl, H. J. Neusser, and E. W. Schlag, Z. Naturforsch. A, 33, 1546 (1978). (4)J. H. Brophy and C. T. Rettner, Chem. Phys. Lett., 67,351(1979). (5)L. Zandee and R. B. Bernstein, J. Chern. Phys., 70,2574 (1979); 71,1359 (1979). (6)D. M.Lubman, R. Naaman, and R. N. Zare, J. Chem. Phys., 72, 3034 (1980). (7)G. Fisanick, T.S. Eichelberger, B. A. Heath, and M. B. Robin, J . Chem. Phys., 72,5571 (1980). (8)T. G. Dietz, M. A. Duncan, M. G. Liverman, and R. E. Smallev, Chern. Phys. Lett., 70, 246 (1980). (9)U.Boesl, H. J. Neusser, and E. W. Schlag, J. Chem. Phys., 72,4327 (1980). (10)D. A. Lichtin, S. Datta-Ghosh, K. R. Newton, and R. B. Bernstein, Chem. Phys. Lett., 75,214 (1980). (11)D. H. Parker and M. A. El-Sayed, Chern. Phys., 42,379 (1979). 0022-3654/81/2085-0015$01 .OO/O
present experiments the second and third harmonics generated from the Nd:YAG laser were used. The second harmonic (532 nm) pumped the dye laser, tuned to 559 nm (R6G dye used), producing 5-ns pulses of this green (G) light. The 5 4 s third harmonic (355 nm) pulses of UV (V) radiation were passed through an optical delay such that they arrived at the ionization region in the TOFMS well after the end of the simultaneously generated G pulse. Measurements made with a fast photodiode at the position of the power meter indicated a 12-11s peak-to-peak difference in the time of arrival of the two pulses. Average laser power levels were measured with a Scientech (volume absorbing disk) calorimeter. The entire optical system for the present experiments was composed of elements external to the TOFMS vacuum system (the 0.1-m fl “internal” lens of ref 10 having been removed). The G beam was reflected by two uncoated prisms, focused by a coated 0.5-m fl lens and finally reflected by a dichroic mirror into the TOFMS ionization region. The 355-nm third harmonic pulse passed through the optical delay and was focused by using a 1.0-m fl quartz lens. After focusing, the pulse was reflected two more times and finally passed through the dichroic mirror to the TOFMS ionization region. The mirrors and prisms were adjusted so that the G and V beams coincided (within 2 mm) at locations 0.3 m before and 1.1 m after the ionization zone. The 0.5-m fl lens was placed 0.49 m in front of the ionization zone, yielding a beam diameter of ca. 0.1 mm in the region, while the 1.0-m fl lens was at a distance of either 0.99 m in one arrangement or 1.87 m in a second arrangement, depending upon the focusing condition desired. Rough estimates of the beam waists in the ionization zone are 0.06 mm for the first arrangement and 2.3 mm for the second. To align the two beams such that they passed coaxially through the ionization region was difficult; the procedure made use of a set of alignment irises through which both beams had to pass. As in ref 10, the Dabco sample was the vapor above a previously degassed solid powder (Pfaltz and Bauer Co. 1,4-diazabicylo[2.2.2]octane). A constant pressure of ca. 0.1 torr was maintained on the high-pressure side of the TOFMS molecular leak inlet system, yielding a nominal pressure (inside the ionization region) of ca. 4 X 10“ torr. The experiments were carried out as described in ref 10, recording and averaging single-segment TOF mass spectra from m / e 15 to 115 for 1000 successive laser shots. Laser pulse energies were set by adjusting the input flash energy and thus the output of the Nd:YAG laser. Three TOF mass spectra would be collected in succession without varying the laser energy, first blocking one color output, 0 1981 American Chemlcal Society
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The Journal of Physical Chemistty, Vol. 85, No. 1, 1981
