J . Phys. Chem. 1987,91, 3 125-3 128
3125
Two-Color Double Resonant Multiphoton Ionization of N, and the LIF Detection of N,+ Ion Produced by Multiphoton Ionization Takayuki Ebata,* Asuka Fujii, and Mitsuo Ito Department of Chemistry, Faculty of Science, Tohoku University, Sendai 980, Japan (Received: March 30, 1987)
Highly excited states of the N2 molecule around 104 000 cm-' have been measured by two-color double resonant MPI via the a'n, state in a supersonic free jet. The (b' and c') 'ZUt states and the (b and c) 'nustates were observed and the spectra show clearly the parity selection rule for the one-photon dipole transition. The relative intensities of the rotational branch of the In, aln,spectra show considerable mixing of the 'nustates with the '2,' state. Also, laser-induced fluorescence detection was applied to measure the Nzt ions produced by the (2 + 2) MPI through the a'n, state at room temperature. It was suggested that the produced N2+ion is rovibrationally highly excited. +-
Introduction The vibronic structure and the intensity distributions of the N 2 molecule in the absorption spectrum starting below X i= 100 nm are very complicated due to vibronic interaction^.'-^ Of special interest is the mixing of the Rydberg states and the valence states. In this energy region two Rydberg states ~'~Z,+(4pa,)and c'llu(3p7r,) are mixed with three valence states b''Zu, b'II,, and olnuby a homogeneous perturbation and quantitative calculation has been done based on the perturbation theory using coupled oscillator differential Besides the one-photon absorption, the resonant-enhanced multiphoton ionization spectra and the photoelectron spectra (PES) were observed around this region by Pratt et aL6 They measured the photoelectron spectra after the (3 1) multiphoton ionization of N2 via the several vibrational levels of the c"Z,+, dn,, b"X,+, and b'n, states. They found that the photoelectron spectra do not agree well with the Franck-Condon factors using adiabatic Morse and RKR potentials and they attributed the discrepancy to vibronic interactions. In the present experiment, we have applied two-color double resonant multiphoton ionization to study these states of N2. The first laser light was fixed to excite N2to a specific rotational level in the a'II,(u=l) state by two-photon absorption. The second laser light was scanned to further excite the a l n q state molecule to highly excited states (in the 104000-cm-' region) and the molecules absorb another photon to ionize. The spectra become very simple compared with other methods, since the one-photon dipole transition between the a'n, state to the upper state is highly restricted. Besides the homogeneous perturbation, considerable coupling (heterogeneous perturbation) was observed between the 'ZU+ and 'nustates, which was found by the anomalous intensity of the rotational structure. We have also measured the laser-induced fluorescence (LIF) spectra of the N2+ion generated by (2 + 2) multiphoton ionization via the aln,(o=l) state in the static gas cell condition at room temperature. Due to the very fast collision-induced relaxation process, we could not detect the nascent internal distribution under completely collision-free condition. However, the measured N2+ ions are rovibrationally highly excited indicating the possibility of the application of MPI to the ion-molecule reaction.
+
Experimental Section The setup for the two-color double resonance MPI combined with a supersonic free jet was described elsewhere.'** N, at a (1) Carrol, P. K., Collins, C. P. Can. J . Phys. 1969, 47, 563. Dressler, K. Can. J. Phys. 1969, 47, 547. Lefebre-Brion, H. Can. J. Phys. 1969, 47, 541. Leoni, N. W. Dissertation NO. 4939, ETH Zurich, 1972. Stahel, D.; Leoni, M., Dressler, K. J . Chem. Phys. 1983, 79, 2541. Pratt, S. T.; Dehmer, P. M.; Dehmer, J . L. J. Chem. Phys. 1984,8/,
(2) (3) (4) (5) (6)
stagnation pressure of 2 atm was expanded into a vacuum chamber (background pressure = 2 X Torr) through a 0.4-mm pulsed nozzle. The jet-cooled molecule was pumped to a specific rovibronic level in the a l n gstate by two-photon absorption of the first laser light ( q ) . The second probe laser light (v2) was introduced coaxially to the first laser beam and excited the a'n, state to higher excited states and the molecule was successively ionized by the absorption of another photon. A Nd:YAG laser (Quantel YG 581-10) was used to pump the two dye lasers simultaneously. The first laser light (vl) is the second harmonics of a dye laser (Quantel TDL 50, rhodamine 6G dye) and produced 2-10 mJ at 283 nm with a line width of 0.2 cm-'. The second laser light (v2) is also the frequency doubled output of another dye laser (Molectron DL14, rhodamine B dye) and produced 0.3 mJ at 295 nm with a line width of 1 cm-'. Both laser beams were introduced coaxially into the ionization region without a delay time. The produced ions are brought into the detector chamber by a repeller at an electric field of 12 V/cm and detected by an electron multiplier (Murata celatron). The signal was amplified by a current amplifier (Keithly 427) and averaged by a boxcar integrator (Par Model 4402/4420). For wavelength calibration, the output of the dye laser was partially reflected and introduced to a gas cell containing iodine vapor at room temperature to measure the LIF spectrum of the iodine molecule simultaneously. The accuracy of the wavelength is determined to be within 0.3 cm-I. In the experiment of the observation of N2+ ion, the same Nd:YAG laser pumped dye laser was used for the first laser light to ionize N 2 by (2 + 2) MPI through the a'II,(v=l) level. The second laser is a nitrogen laser (Molectron UV 24) pumped dye laser (DL14, BBQ dye) for probing the N2+ ion by the B2Z,+(u') X2Zgt(v'') transition. The delay time between the first and second lasers was set by a digital delay line (BNC Model AP-3). The delay time was determined by monitoring the two laser outputs by a fast photodiode (Hamamatsu sl190) and monitored by a digital oscilloscope (Sony Tektronics 2430). Nitrogen was flowed in a static gas cell at a pressure of 1-10 Torr and the pressure was measured by a capacitance manometer (MKS Baratron). Both laser beams are introduced coaxially and fluorescence from the B2Z,+ state of the N2+ ion was measured at a right angle to the laser beams by a photomultiplier (Hamamatsu R-928) after passing through a filter (Toshiba Y42 and V40). The photocurrent was averaged by the same boxcar integrator and recorded on a chart recorder. +-
Results and Discussion Highly Excited States of N2. Figure l a shows the (2 + 2) MPI
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spectrum of jet-cooled nitrogen due to the transition a'rIg(d= 1) X'Z,+(u"=O), which was measured 10 mm downstream. The
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(7) Ebata, T.; Anazaki, Y.; Fujii, M.; Mikami, N.; Ito, M. J . Chem. Phys. 1985.87.4773.
0022-3654187'1209 1-3125$01.50 ,IO ,
I
(8) Ebata, T.; Anezaki, Y.; Fujii, M.; Mikami, N.; Ito, M. Chem. Phys. 1984, 84, 15 1 .
0 1987 American Chemical Society
Letters
3126 The Journal of Physical Chemistry, Vol. 91, No. 12, 1987 6
5
0 1234
4 3 2
I
t
I
I
1
n
I
I
2 83.4
I
1
283.2 LASER WAVELENGTH / n m
283.0
I
1
I
34000
33500 V2 I
I
70560
I
I
I
70600
70640
TOTAL WAVEN UMBER / c m - ' Figure 1. Two-photon resonant four-photon ( 2 2) MPI spectrum of N, a'n,(u=l) XIZ,+(u=O): (a) observed in a supersonic free jet IO-" downstream and (b) calculated by assuming T, = 25 K, see text.
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+
TABLE I: Band Origins E (cm-I) and Rotational Constants B (cm-I) Observed in the 104 400-cm-I Region Enhda Eb AE Bown Bb AB c'II,(u=O) 104133.5 104138.7 -5.2 c'Z,+(U=O) 104323.0 104323.3 -0.3 b'lZu+(u=l) 104416.0 104418.7 -2.7 b1II,(u=5) 104699.3 104700.2 -0.9 a This
work.
1.505 1.907 1.164 1.449
1.497 0.008 1.929 -0.022 1.150 0.014 1.433 0.016
From ref 5
two-photon absorption spectra of the alII,(u') XIZ,+(u"=O) transition were measured by several workerse" and the spectrum can be assigned by five rotational branches. The intensities of the indivdual rotational lines are given by Ij,jrc = Agjwsjtjr, exp(-E,,,/ kn +-
where A is a constant, gJtt is the statistical weight due to nuclear '~ spin, and Sj*j,tis the two-photon Honl-London f a c t ~ r . ' ~ ,The calculated spectrum is also shown in Figure 1b when the rotational temperature is 25 K . Two-color double resonant MPI spectra were obtained by fixing the first laser light on each rotational line of the a'II,(u'= 1) X'Z,+(u''=O) transition and then by scanning the second laser
+-
(9) Van Veen, N.; Brewer, P.; Das, P.; Bersohn, R. J . Chem. Phys. 1982, 77, 4326. (10) Helvajian, H.; Dekoven, B. M.; Baronavski, A. P. Chem. Phys. 1984, 90, 175. (1 1 ) Carleton, K. L.; Welge, K. H.; Leone, S. R. Chem. Phys. Left. 1985, 115, 492. (12) Bray, R. G.; Hochstrasser, R. M. Mol. Phys. 1976, 31, 1199. (13) Chen, K. M.; Yeung, E. S. J . Chem. Phys. 1978, 69, 43.
W A V E N U M B E R / cm-'
Figure 2. Two-color double resonant MPI spectra of N2 (b) from the J = 2(+) level ( Y , is fixed on the S(0) line, 70626.8 cm-') and (c) from the J = 2(-) level (uI is fixed on the P(3) line, 70603.0 cm-') of the a'n,(u=l) state. The broken curve (a) is the probing laser ( u z ) power.
light in the 33 400-34 100-cm-' region (total energy is from 104000 to 104700 cm-'). Figure 2 shows the typical double resonant MPI spectra. Four transitions were observed in this energy region and were assigned to two Rydberg states c'II,(u=O) and c'2+,(u=O) and two valence states b111,(u=5) and b'Z+,(u= l ) . The energies of the band origin and the rotational constants are listed in Table I together with the values obtained by onephoton absorption from the ground state. Figure 2a shows the intensity curve of the second laser (vz). In Figure 2b, v 1 is fixed on the S(0) line, while in Figure 2c, v1 is fixed on the P(3) line. Therefore, both spectra show the transition from the same J' = 2 rotational level of the aIII,(u=l) state but having different parity. Interesting features can be seen between the two spectra because of the parity selection rule. Due to the one-photon parity selection -, the Z+, II, transition shows only P and R branches rule in Figure 2b, while in Figure 2c only the Q branch appears. This can be easily understood by the Herzberg diagram in Figure 3a. Similar parity selectivity is reported for the BIZ+ AIII transition of CO by S H A et a1.14 and by Klopotek et The 'II, a'II,(u=l) transitions at 33 500 and 34200 cm-I region have essentially three rotational lines P, Q,and R, which is understandable since each J has two (+ and - parity) levels states. However, we can see corresponding to the nu+and nustill the difference between the two spectra. In the transition from the + parity level of J = 2 (a'II,), the P and R branches are ten times stronger than the Q branch (Figure 2b), while from the parity level these three branches have comparable intensities (Figure 2c). The Honl-London factor from the J = 2 level for the II-n transition is 2.7 for the R branch, 0.83 for the Q branch,
+
-
-
-
-
(14) Sha, G.; Zhong, X.; Zhao, S.;Zhang, C. Chem. Phys. Leu. 1984,110, 405. (15) Klopotek, P.; Vidal, C. R . J . Opt. Sor. Am. 1985, B2, 869.
The Journal of Physical Chemistry, Vol. 91, No. 12, I987
Letters
3127
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Figure 4. LIF spectra of the N2+ion (B2Z,+, u X2Zg+,u ) produced by (2 2) MPI via the a'n,(v=l) state of N2. The delay time between the probing laser and the ionizing laser is 40 ns. The upper spectrum was measured at PN2= 10 Torr and lower at PN2= 1.0 Torr. The broken
+
line is the probing laser power.
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TABLE II: Franck-Condon Factors for the Transition N2+X2Zg+(v ) N2a'n,(v=l)and Nz+ B22,+(v) XzZ,+(v)"and the Observed Relative Fluorescence Intensity at PN,= 1 Torr
0
N2 aln,(u=l) N2+ B2Z,+(v) ----$---~-8----8
2
3
5 Figure 3. Herzberg diagram showing the double resonance selection rule: a'n, X%,+ transition and (b) In, aln, X'Z,+ (a) '2,' transition. The dotted lines indicate the interaction between the lZU+and 'nustates. J
- -
O
1
+-
4
- --
and 1.5 for the P branch and these values do not depend on the parity. The intensity distribution of either one of the spectra of Figure 2 does not agree with the expected one. This discrepancy clearly indicates mixing of the I' component of each II, state with the neighboring c'Z,+(v=O) state, so called heterogeneous a'II,(v=l) perturbation. Since the intensity of the C"Z,+(v=O) transition is strong, the In,+ a'II,(u=l) transition will be more alII,(v=l) transition due to intensity intense than the Ill,,borrowing by mixing with the C'~Z,,+(U=O)state (see the Herzberg diagram in Figure 3b). In this case we will find stronger P and R branches and a weaker Q branch than the one expected by the Hod-London factor in the excitation from the J = 2(+) level. On the other hand, in excitation from the J = 2(-) level the intensity of the Q branch should be comparable to the P and R branches. The measured spectra shown in Figure 2 clearly meet our expectation and from the relative intensity the 'II, states are thought to be mixed considerably with the c''Zu+ state. Up to now, several calculations were done for the mixing of the Rydberg and valence states by solving coupled oscillator differential equations for the (b' + c' + e')IZ,+ states and the (b c O)IIIu state^.^^^ However, their calculations were done separately for the lZu+states and the Ill, states and did not include mixing between both sets of states. The present results have revealed considerable mixing between the IZ,+and In, states and we emphasize that this type of mixing was found for the first time by the parity selection achieved by two-color double resonance spectroscopy. Observation of the N2+Zon after MPZ. The final goal of this experiment is to measure the nascent internal (vibrational and rotational) distribution of the N2+ ion which is produced by resonance-enhanced multiphoton ionization by the laser-induced fluorescence method. Especially for the rotational distribution, the L I F method has an advantage over a photoelectron measurement because of the capability of high-energy resolution.
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+ +
-
+-
0.215
productb
0.669 1.0
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N2+ X2Z,+(u) 1 2 0.155 2.43 X lo4 0.252 7.31 X 0.27 1.23 X lo4 0.33 -0.1
3 0.122 0.241 0.2
'Only the diagonal transition (Au = 0) is listed. bThe values are normalized so that the u = 0 intensity equals one. Figure 4 shows the laser-induced fluorescence excitation spectra of N2+measured 40 ns after the 2 + 2 ionization through the overlapped S(2) and S(13) lines of the alIIg(u'= 1) X'Z,+(v"=O) transition at several vapor pressures, where the S(2) line is around three times stronger than the S( 13) line. The spectra in this region is the diagonal part (for example, (O,O), ( l , l ) , etc.) absorption. As seen in Figure 4, at 1 Torr of the B2Z,+ X2Z8+ of N2,the initially prepared rotational levels after the ionization are already redistributed and the vibrational distribution of N2+ was seen up to u = 2. With increasing pressure, a fast vibrational relaxation occurs and at 10 Torr, the ion is populated only in u = 0 but is rotationally still hot. The rotational distribution can be expressed by the sum of two Boltzmann distributions of T, = 400 K and T, = 900 K. Table I1 lists the Franck-Condon factors for the transitions from the a'II,(u=l) state of N2 to the X2Zg+(v)state of the N2+ion with u = 0 to u = 3 and also for the transitions between the B2Zu+(v') and X*Z,+(u'') states of the N2+ion. The FranckCondon factors were obtained by assuming Morse potentials for these states using the constants given by Huber and Herzberg.I6 The product of the Franck-Condon factors for the two transitions can be compared with the observed laser-induced fluorescence intensity. Due to the wide spread rotational distribution it is difficult to obtain an accurate vibrational distribution. However, disagreement between the calculated and observed intensity distributions is clear for D = 2 level even at 1 Torr (see Table II), indicating that even at this pressure the vibrational relaxation is extremely fast. The high LIF intensity of u = 2 may come from the higher vibrational level ions by collision. Energetically, the N2+ion is produced up to the u = 7 level by the (2 + 2) MPI via the u = 1 level of the a'II, state. The quenching rate constant of the vibrationally excited state of N2+ion is reported" to be 5 X 1O-Io cm3 molecule-' s-l, which is almost equal to the Langevin value of 8.0 X 1O-Iocm3 molecule-'
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+ -
(16) Huber, K. P.; Herzberg, G . Molecular Spectra and Molecular Structure, Vol. IV, Van Nostrand-Reinhold: New York, 1979. (17) Ferguson, E.E. J . Phys. Chem. 1986, 90, 731.
J. Phys. Chem. 1987, 91, 3 128-3 13 1
3128
7 simultaneously, we cannot discuss much about the relaxation and as a next step we are planning to apply the two-color double resonance MPI method for the production of the N2+ion in a single rovibronic state.
s-'. This fast relaxation comes from the charge-transfer reaction. At a delay time of 40 ns, the pressure of 1 Torr is still high and around 50% of the N2+ions experience collision before the detection and therefore we have not measured the nascent rovibrational distribution of the N2+ion produced by MPI. We could not measure the spectra of the N2+ion below 1 Torr under present conditions. Since the Nz+ ions are produced from v = 0 to v =
Acknowledgment. The authors express their sincere thanks to Dr. N. Mikami for experimental support.
Laser Photolysis, Infrared Fluorescence Determination of CH3(v3)Vibrational Deactivation by He, Ar, N,, CO, SF,, and (CH,),CO D. J. Donaldson and Stephen R. Leone*+ Joint Institute for Laboratory Astrophysics, National Bureau of Standards and University of Colorado, and Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0440 (Received: April 2, 1987)
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Room temperature vibrational deactivation rate constants are reported for methyl radicals with antisymmetric stretch excitation, CH,(v,) + M CH, + M, where M = He, Ar, Nz, CO, SF6, (CH3)2C0. Excimer laser photolysis of acetone at 193 nm is used to populate CH,(v,), and time-resolved infrared emission from the CH stretch is used to follow the deactivation kinetics. The rate constants obtained are ( f 2 u ) (2.6 f 0.5) X lo-'' (He), (6.8 f 0.7) X IO-', (Ar), (6.1 f 0.6) X lo-', (Nz), (3.6 i 0.7) X lo-'' (CO), (6.9 f 0.7) X lo-'' (SF,), and (8.1 f 0.9) X (CH3COCH3)in units of cm' molecule-' s-'. The deactivationprobability is not controlled by long-rangeforces due to the lone electron on the radical, but rather by the probabilities for intramode vibrational energy flow in CH3.
Introduction The study of vibrational deactivation of open-shell species is of great importance, both as a test for energy-transfer theories and because of the considerable effect that internal excitation has on the kinetics of elementary reactions.',2 A complete understanding of complex reaction systems therefore demands a thorough knowledge of the relevant energy-transfer processes as well as the reactive ones. Available results for vibrational deactivation of open-shell radical species are very limited. The transient chemical nature of radicals makes detailed measurements of their internal state distributions difficult. For this reason, measurements have been confined almost exclusively to polyatomic free radicals which are formed photolytically in fortuitous, well-defined internal states. The time evolution of these states is then followed in the presence of a variety of collision partners. Recent studies of this type include the relaxation of H C O (O,l,O),, N H z (0,1,0),4 and C H , ( V ~ ) . ~ The unpaired electron of free radicals is expected to influence the collision dynamics greatly. Inelastic processes which are facilitated by formation of collision complexes will be aided by the ability of the unpaired electron to form transient bonds with other species. This undoubtedly accounts for the efficiency with which HCI, NO, and O2are deactivated by halogen and oxygen atoms.6 Complex formation is also thought to be important in the deactivation of HCO(010) by O2 and NO.' These species are 100 times more effective at removing HCO(010) than is N2.' In the present study we use the technique of laser photolysis, infrared fluorescence to measure the deactivation rates of CH, excited in the v3 antisymmetric C H stretch mode. The photolysis of acetone at 193 nm produces methyl radicals with substantial excitation in this mode, which has been shown to be primarily in v3 = l ? By measuring the decay of the infrared emission intensity from these radicals as a function of time with a variety of collision partners, energy-transfer rates are obtained. We have recently
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Staff member, Quantum Physics Division, National Bureau of Standards, Boulder, CO 80309
0022-3654/87/2091-3 128$01.50/0
used a variant on this method to measure the rate constants for methyl radical reaction with HBr and HI* and C12 and Br2.9
Experimental Section The rate constants are measured in a flow cell, described is introduced in the first part of the cell p r e v i o ~ s l y .Acetone ~~~ through a pinhole reagent inlet directed backward into a flow of 100-1500 Pa (1 Pa = 7.5 X lo-, Torr) of deactivator gas. A valve on the exit side of the cell allows the total pressure to be adjusted while keeping the flow rate constant. The acetone (10-20 Pa) is photolyzed in the central portion of the cell by the 193 nm output of an ArF excimer laser (20-11s pulses, 5-40 mJ/pulse) operating at 10 Hz. The vibrationally excited methyl radicals formed in the photolysis are deactivated via collisions with the deactivator gas and the residual, unphotolyzed acetone still present in the cell. The concentration of vibrationally excited CH,(v,) is proportional to the infrared emission intensity from the radical. The time dependence of the infrared emission intensity is measured with a large-area InSb detector mounted directly above the photolysis region. A narrow band-pass interference filter with a transmission maximum at 3.47 pm and a fwhm of 0.54 Nm allows emission from only the C H stretching region of the spectrum to reach the detector. The resulting signals are amplified in two stages, digitized by a 100-MHz transient digitizer, and summed in a signal averager. The response time of the detector and associated electronics is -1 ps. Typically, the results of (1) Smith, I. W. M. Chem. SOC.Reo. 1985, 14, 185. (2) Wolfrum, J. In Reactions of Small Transient Species; Fontijn, A,, Clyne, M. A. A., Eds.; Academic: New York, 1983. (3) Langford, A. 0.;Moore, C. B. J . Chem. Phys. 1984,80,4204,421 I . (4) (a) Xiang, T.-X.; Gericke, K.-H.; Torres,L. M.; Guillory, W. A. Chem. Phys. 1986,101, 157. (b) Gericke, K.-H.; Torres, L. M.; Guillory, W. A. J . Chem. Phys. 1984,80,6134. ( 5 ) Callear, A. B.; Van den Bergh, H.E. Chem. Phys. L e r r . 1970, 5, 23. (6) Kneba, M.; Wolfrum, J. Annu. Reu. Phys. Chem. 1980, 32, 47 and references therein. (7) Donaldson, D. J.; Leone, S. R. J . Chem. Phys. 1986, 85, 817. (8) Donaldson, D. J.; Leone, S. R. J . Phys. Chem. 1986, 90, 936. (9) Kovalenko, L. J.; Leone, S. R. J . Chem. Phys. 1984, 80,3656
0 1987 American Chemical Society
