- Email: [email protected]
Photoionization of highly vibrationally excited ground electronic state azulene near the ionization threshold
Volume 144, number 4
CHEMICAL PHYSICS LETTERS
4 March 1988
PHOTOIONIZATION OF HIGHLY VIBRATIONALLY EXCITED GROUND ELECTRONIC STATE AZULENE NEAR THE IONIZATION THRESHOLD
Joseph E. SABOL and Robert W. CARR Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, USA
Received 3 November 1987; in final form 28 December 1987
Highly vibrationally excited ground electronic state azulene, SE, was formed at N, laser excitation of SZat 337,1 nm followed by internal conversion, and photoionized by a time-delayed pulsed dye laser. Ion currents were measured with a parallel plate ion chamber. Photoionization cross sections were estimated by using rate equations to describe ion pmduction rates. The cross sections are 3 X 1O-22cm’ for photoionization by 30303 cm-’ photons, and 6x lO-Z’cm2 by 31153 cm-’ photons.
1. Introduction
Spectroscopic detection of highly vibrationally excited polyatomic molecules presents special problems. For species with vibrational excitation in the range of chemical bond energies, the vibrational state density is usually exceedingly large. Consequently, electronic spectra consist of broad, diffuse bands, and vibrational spectra are poorly understood. Furthermore, lifetimes due to unimolecular reaction and collisional deactivation are typically in the microsecond to nanosecond or even subnanosecond range. Pulsed laser excitation has been used to produce sufficiently large transient populations of highly vibrationally excited ground electronic state molecules to permit direct detection in recent years. Since the broadened and red-shifted (with respect to cold molecules) UV absorption spectra of photoactivated methyl- and ethyl-cycloheptatriene were first reported [ 11, UV absorption spectroscopy has been used to further investigate the cycloheptatrienes [ 2-41, as well as vibrationally hot toluene [ 5-71, CF31 [ 81, benzene [9] and hexafluorobenzene [ 10-121. Detection of photoactivated azulene via infrared emission arising from upper vibrational levels has been employed by Barker and co-workers to follow excited state dynamics [ 13- 181. Although much of the research on multiphoton ionization has been oriented toward investigations of the spectra of both resonant intermediate states
and of ions, the determination of ionization threshold energies, and in combination with mass spectrometric detection to study photoionization and fragmentation mechanisms, it can also be used as a probe to investigate excited state kinetics. Two-color experiments in which the photoionization probe pulse is time delayed with respect to the pump pulse have been used to monitor excited state dynamics. This technique has been successfully used to follow the decay rates of electronically excited molecules [ 19,201. Detection of vibrationally excited ground electronic states has also been reported [21,22]. In two-color pulsed laser photoionization of methylcycloheptatriene with variable time delay between the pulses, Borrell, Lijhmansroben, and Luther [ 221 observed a time-dependent photoion current which was attributed to hot photoproducts that were at least partially relaxed. We report here two-color pulsed laser photoionization of azulene. Highly vibrationally excited ground electronic state azulene (S$) formed by internal conversion of SZ azulene after 337.1 nm excitation, is photoionized by 330 or 321 nm photons which are delayed by a time interval long enough for population of the ground state to be virtually complete, but short enough that it does not, on average, collide before photoionization. The two-photon energy is just greater than the ionization threshold. Photoionization cross sections are small, presumably due to unfavorable Franck-Condon factors.
0 009-26141881%03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
401
Volume 144, number 4
CHEMICAL PHYSICS LETTERS
Azulene is an excellent source of vibrationally hot ground state molecules. The second excited state, which can be conveniently pumped with an Nz laser at 337.1 nm undergoes rapid and efficient internal conversion to the ground state. Fluorescence occurs only to So, with a quantum yield, &=0.03 1 in cyclohexane [ 231, and Woudenberg et al. have concluded from experiments with jet-cooled azulene that intersystem crossing is absent in azulene vapor [ 241. Internal conversion to So occurs both directly and via S,, the former path increasing in importance with increasing excitation energy [25]. Also, the quantum yield for photoisomerization to naphthalene (313nm,0.01 Torr, 300K)is0.5~10-5 [26].Thus, in azulene vapor, the quantum yield for SE formation when excited via SZ is about 0.97. The S2 fluorescence lifetime at 337.1 nm from nanosecond flashlamp excitation has been reported to be 2.09 ns [27]. Recent measurements of the energy dependence of rf( S,) by Demmer et al. [ 281 following picosecond excitation indicate that the lifetime is significantly longer than this. They reported r,( 340 nm) ~3.2 ns and zf( 332 nm) =2.6 ns.
2. Experimental The photoionization apparatus is shown in fig. 1. The ionization chamber is a stainless steel cylinder, 64 mm diameter% 127 mm, fitted with a 25 mm diameter quartz window at each end and two ceramic high vacuum electrical feedthroughs, arranged diametrically at the center of the cylinder wall. A small detachable sample reservoir could be isolated by the
Fig. 1, Photoionization experiment showing excitation/ionization lasers, optical delay path, photoionization chamber, and detection electrmrks. 402
4 March 1988
use of a high vacuum bellows valve. The ion chamber was attached through another bellows valve to a conventional borosilicate 7740 glass vacuum manifold equipped with a Hg diffusion pump, McLeod gauge, and 10d3 to 10 Torr pressure transducer accurate to 0.1 mTorr. A vacuum of 10m6Torr could be maintained on a regular basis. A 28 mmx 115 mm stainless steel repeller plate was connected through one of the ceramic feedthroughs and biased at -45 V to ground, at which voltage the ion current was determined to be independent of the applied field. A stainless steel collector plate, 12.5 mm x 100 mm, was connected to the other ceramic feedthrough and surrounded by a 17.5 mmx 105 mm (inside) by 28 mmx 115 mm (outside) guard electrode which was spot welded to the chamber wall and held at ground potential. This arrangement of the guard electrode ensured a uniform electric field perpendicular to the collector electrode. The separation between the repeller and collector plates was 42 mm. The collector plate was connected by a short length of RG-58/U coaxial cable to the input of a low-noise LHO062 transimpedance preamplifier, gain 10’V/A. The preamplifier output could be observed directly, or amplified 100x by a voltage follower, with the resulting total gain being 10’V/A. The RC time constant of the feedback loop was adjusted so the circuit could reproduce a calibration square wave of 1 ps risetime. Extensive care was taken in the physical layout of the ion chamber/detector/preamplifier in order to minimize the input capacitance. A Faraday cage consisting of two layers of fine mesh brass screen was built around the ion chamber/detector/preamplifier to shield out any RF1 broadcast by the high voltage laser trigger circuit. Single shot waveforms of the amplified ion current were observed and photographed from an oscilloscope, while signal averaging was accomplished by the use of a boxcar averager. The excitation source was a N2 laser producing 10 mJ pulses of 7 ns fwhm duration. A portion of this beam was split off with a quartz microscope slide and passed through a 1 cm diameter aperture prior to entering the cell. High speed silicon photodiodes were used to trigger the electronics form the Nz laser pulse and to monitor the relative pulse to pulse intensity. Typical pulse energy, measured with a pyroelectric
Volume 144, number 4
4 March 1988
CHEMICAL PHYSICS LETTERS
energy/power meter, was 50 l.tJ. The remainder of the nitrogen beam was used to pump a tunable rhodamine 640 dye laser which had a bandwidth of 0.01 nm. Frequency doubling 2v of the dye fundamental v was accomplished with an angle-tuned KDP crystal. The doubled dye pulse energy was typically 10 pJ. The unwanted portion v of the v+2v beam was removed by use of an UV passing filter or a 0.4 m monochromator. The 2v beam entered the photoionization chamber from the end opposite the nitrogen pump beam entrance. An optical delay path separated the pump and photoionization pulses by 20 ns. The pulse separation was verified with a fast silicon photodiode, observing the resulting waveform with the aid of a 2 ns resolution transient digitizer. Azulene was obtained from Aldrich Chemical Company and was purified by repeatedly pumping away the vapor. All experiments reported here were done at a pressure of 20 mTorr of azulene.
L 0
3. Results
The vertical ionization energy of azulene is 7.41 eV [ 29,301. Thus, when excited into SZ at 29665 cm-’ with the N, laser, azulene requires a minimum of 30 110 cm- ’ additional energy to reach the ionization threshold. To ensure that the total energy was above the threshold, the dye laser was tuned to produce a frequency-doubled pulse of either 30303, or 31153 cm-’ in two-color photoionization experiments. Typical ion current waveforms are shown in fig. 2. Photoionization was observed in onscolor photoionization with the N2 laser, fig. 2a. The energy of two 29665 cm-’ photons is 7.35 eV, which is approximately 0.05 eV below the ionization threshold, Ionization caused by three or more photons is most probably not the cause of this signal since a study of the total charge produced, q, as a function of NZ laser intensity I showed that the slope of a plot of log q versus log I is 2.OkO.1, and the rate of the So-S2 transition is not expected to be much faster than transitions from Sz and higher states. The total number of positive charges produced is obtained by integrating the current waveform,
50
Time, Microsecond Fig. 2. (a) Photoion current from two-photon, one-color (337.1 nm) excitation. The total number of ions produced is obtained from the integrated photoion current, q=Ji dt. The area shown herecorrespondsto (1.8+0.2)~10’ionsor ~3xlO-‘~C. (b) Detector/amplifier output from two-photon, one-color (330.0 nm) excitation, essentially base line. (c) Photoion current from two-photon, two-color (337.1 nm + 330.0 nm, 20 ns delay) excitation. Note the signal from the two-photon, one-color (a) excitation is still present, and the total photoion current is enhanced by the 330.0 nm ionization of St.
CQ
4=
s
idt.
0
Thus, photoionization must be aided by thermal energy. At room temperature 14% of the azulene molecules have more than 0.05 eV of thermal energy which is sufficient, with the addition of two 29665 cm-l photons, to take them to the ionization threshold. The waveforms displayed in fig. 2 are due to transport of azulene ions to the electrodes. The ion transit time can be estimated by the equation t=L21mV, 403
Volume 144, number 4
CHEMICALPHYSICS LETTERS
where L is the interelectrode spacing, m is the ion mobility and Vis the applied voltage [ 3 11. Since no report of the azulene ion mobility was found in the literature the value 0.2 cm2/V s at 1 atm was estimated from data for other species [ 321. Ion mobility scales inversely with pressure. A pressure of 0.02 Torr gives a mobility of 7.6 x lo3 cm2/V s and a transit time of 5.2 X lOa s. This is remarkably good agreement with the 50 its times observed. The electron transit time was estimated to be approximately 1O-’ s, too fast for the microsecond risetime of the detection circuitry, and waveforms due to electrons were not observed. Due to the relatively slow transport of ions, these ion currents contain no information on the much faster azulene photophysics. The positive ion yield obtained by integrating the photoion current waveform gives the total photoion yield from all photoionization paths. Fig. 2b shows that no photoion current was detected when azulene was exposed to only the dye laser (N2 laser beam blocked) while f’ig.2c shows results with the N2 laser and the 20 ns delayed pulse from the dye laser. Laser operating conditions were held constant during each set of experiments, figs. 2a-2c. The current waveform was repetitively scanned by the boxcar to average pulse to pulse variations in laser energy. The absorption of two 30303 cm-’ photons, fig. 2b, from the dye laser result in azulene with 7.5 1 eV, well above the ionization threshold. The combination of modest dye laser energy (10 pJ) and low absorption by azulene at this wavelength produces ion currents too small for detection with the present ion chamber/detector/preamplifier. Fig. 2c shows the current waveform resulting from excitation by the 29665 cm-’ pulse followed (separated in time but overlapped in space) by the 30303 cm-’ pulse, total energy 7.43 eV. The amplitude of the waveform in fig. 2a is clearly greater than the amplitude shown in fig, 2a. Furthermore, integration of the curves revealed that the number of ions produced using both lasers is larger than the number formed from the N2 laser alone. Since the enhanced signals in the twocolor time-delayed experiment are only slightly greater than the sum of the one-color ion currents from figs. 2a and 2b, the experiment was repeated several times with the dye laser at 30303 cm-‘, and also at 31153 cm-‘. These data are summarized in 404
4 March 1988
Table 1 Number of ions produced under various photoionization conditions Excitation (cm-‘)
Number of ions
Charge (C)
29665 30303 29665+30303 enhancement (29665+30303)-29665 31153 29665+31153 enhancement (29665+31153)-31153
1.8~10~
2.9x lo-l4
2.0x 105
3.2x lo-l4
0.2~10~ 4.6~10~ 8.2~10’
0.3x lo-l4 7.4x lo-l4 1.3x 10-I’
3.6x10’
5.9x 10-14
table 1. In the case of the one-color two-photon 3 1153 cm-’ excitation, azulene containing 7.72 eV is formed and substantial ion current was observed. Nevertheless, the ion signal is enhanced when the N2 laser pulse precedes the dye laser pulse.
4. Discussion The one-color photoionization of azulene by the N2 laser is a resonance-enhanced two-photon ionization via SZ. In the two-color experiments this signal is still present, and the additional ions created are due to photoionization induced by the dye laser. At 20 mTorr total pressure, the collision interval is significantly longer than the delay time between the laser pulses, and the experiment can safely be considered to occur in the absence of collisions. When the dye laser pulse arrives, the SZpopulation has decayed to less than 0.1% of its peak value. Thus, the dye laser interrogates an excited state population dominated by St with 29665 cm-’ of vibrational energy. Experiments done with pure naphthalene (S, origin 32018 cm-‘, IP 8.13 eV) samples at pressures of approximately 100 mTorr gave photoion currents approximately 60% smaller than the azulene signals. Since naphthalene yields from photoisomerization of azulene are expected to be small ( Q: 1 mTorr), the contribution of the ion signals from naphthalene is negligible. An estimate of the cross section for photoionization of S$ was made by obtaining an approximate solution to the set of coupled differential equations describing the applicable azulene excited state ki-
CHEMICAL PHYSICS LETTERS
Volume 144. number 4
netics. The combination of small laser pulse energy, and optically thin samples provide appropriate conditions for the application of rate equations [ 33,341. Azulene photoionization can be described by the following mechanism: hv(29665 cm-‘) +S0+S2,
~=~02~,(0&,
SZ-&+hv’,
r=kfS, >
s24I:
r=kS2
3
hv(29665 cm-‘)+!$+A:
+e-,
,
T=[TZiZi(f)SZ,
4 March 1988
as confirming the weak excitation condition for using rate equations. The rate of ion formation in the one-color photoionization is ti:ldt=oziZi&(t)f,
(3)
wherefis the fraction of S2 azulenes with enough energy to be photoionized with another N2 laser photon. At 300 Kf= 0.145. Since ions are only produced when the laser is on, S2( t) can be expressed by eq. (2). Integrating eq. (3) using the initial condition A: =O at t=O gives
hv(30303 cm-‘) +S$-+A,+ +e- , r=ooiZ2(t)Sg, The (T’Sare photoabsorption cross sections, Z,(t) and Z2(t) are the time-dependent intensities (photons/cm* s) of the N2 laser and dye laser, respectively, A+ is the azulene photoion, e- the photoelectron, and r the rate per unit volume. The rate of formation of S2 is given by dS,ldt=o,,Z,(t)S,,-
[~+k,,+a2iZ,(t)]S2
.
(1)
In eq. (1) , CziZ,(t) can be neglected as it turns out to be about three orders of magnitude smaller than k, + k,,. The value of Z++kIc= l/t used here is 3.1 x lo8 s- ‘, interpolated from data on the energy dependence of the S, lifetime reported in ref. [ 281. Also, uo2= 5.4x10-18cm2wasused[15].Intheintegration of eq. (1) and other differential equations below a square wave approximation to both I, (t) and Z2(t) is used. While this is an oversimplification of the actual laser pulse shape, it has the virtue that analytical solutions are obtained, while not introducing large errors into the calculated photoionization cross sections. The solution of (1) for the initial condition S,(O) =O is S2(t)=doZZ,S07[1-exp(
-t/z)]
,
(2)
where II = 0 at t < 0 and t > 10 ns; I, = constant in the range 0 < t6 10 ns. Thus eq. (2) describes S2( t) only during the interval during which the pulse occurs. Thereafter S,(t) is described by an exponential decay, S2(t)=S2(t=10 ns) exp[-(t-10)/r], where S2( t = 10 ns) is calculated by eq. (2). The steady state value of S,, obtained when Z,(t) is a step function, is given as t-m by S,,= c~~~Z,S,,~. At 10 ns, S2( t) has risen to 95% of S,,. Furthermore, S2JS0= 3 x 10-3, which justifies S,= constant in integrating (1) , as well
A: =~O2U2iZ~SOfZ{t-r[l-exp(-t/r)]}.
(4)
Using measured values of A : with the N2 laser alone (table 1 ), eq. (4) can be used to calculate g2i, the cross section for photoionization of Sz. For t = 10 ns, 62i= 1.7X 10W20cm2. The cross section for photoionization of Sz can be obtained by considering the rate of production of ions from Sz. The ion signal, A:, in the two-color photoionization corresponds to the enhancement (table l), dA,fldt=o,,Z,(t)S;:.
(5)
Since S2( t) has virtually completely decayed by the time the dye laser pulse arrives, S$ will be stationary in the absence of collisions and other removal paths, and can be regarded as a constant in the integration of (5). Furthermore, Sz will correspond to 97% of the ground state depletion, -dSoldt=ao2L,(t)So.
(6)
Integrating (6) over the 10 ns N2 laser pulse, AS,,=3,78x 10” cm-3. Letting St=0.97ASo and integrating (5) over the 7 ns dye laser pulse gives AZ =ooiZ2(t)S$Ats Using the enhancement of ion current from table 1, we calculate hoi= 3.3 x 10TL2cm2 with the dye laser tuned to 30303 cm-‘, and aoi=6.0XlO-*’ cmm2 with the dye laser tuned to 3 1153 cm- ‘. While these cross sections must be regarded as approximate, the uncertainties introduced by idealization of the laser pulse shape are probably not large, and the calculations are expected to reveal the correct magnitudes of the cross sections. The very small cross sections for ionization of Sz are not unex405
Volume 144, number 4
CHEMICAL PHYSICS LETTERS
petted since the Franck-Condon factors for excitation of SE, which contains approximately 90 kcal/mol of vibrational energy, to the vibrationally cold ion are expected to be very unfavorable [ 351. Also, the relatively small cross section for ionization of SZ, w 10V20cm2, may be indicative of bond length differences between the SZstate of azulene and the molecular ion. Lubman et al., in their study of azulene photoionization, found no ionization in two-color experiments with the beams spatially overlapped but timedelayed [ 361. This is not in conflict with the results reported here since: (1) the excitation mechanism was different and (2) the detector (mass spectrometer-channeltron) used probably has significantly less absolute sensitivity than our ion chamber.
Acknowledgement Acknowledgement is made to the Donors of the Petroleum Research Fund, administered by the American Chemical Society, for the support of this research.
References [ I] H. Hippler, K. Luther, J. Troe and R. Walsh, J. Chem. Phys. 68 (1978) 323. [2] H. Hippler, K. Luther and J. Troe, Faraday Discussions Chem. Sot. 67 (1979) 173. [ 31 H. Hippler, J. Troe and H.J. Wendelken, J. Chem. Phys. 78 (1983) 6718. [ 41 II. Hippler, K. Luther, J. Troe and H.J. Wendelken, J. Chem. Phys.79 (1983) 239. [ 51H. Hippler, J. Troe and H.J. Wendelken, Chem. Phys..Letters84 (1981) 257. [ 61 H. Hippler, V. Schubert, J. Troe and H.J. Wendelken, Chem. Phys. Letters 84 (1981) 253. [ 71 H. Hippler, J. Troe and H.J. Wendelken, J. Chem. Phys. 78 (1983) 6709. [g] Yu.A. Kudriatsev and V.S. Letokhov, Chem. Phys. 50 (1980) 353. (91 N, Nakashima and K. Yoshihara, J. Chem. Phys. 79 (1983) 2727. [lo] T. Ichimura, Y. Mori, N. Nakashima and K. Yoshihara, Chem. Phys. Letters 104 (1984) 533.
406
4 March 1988
[ 111 T. Ichimura, Y. Mori, N. Nakashima and K. Yoshihara, J. Chem. Phys. 83 (1985) 117. [ 121 T. Ichimura, M. Takahashi and Y. Mori, Chem. Phys. I 14 (1987) 111. [ 13 ] G.P. Smith and J.R. Barker, Chem. Phys. Letters 78 (198 1) 253. [ 141 M.J. Rosi and J.R. Barker, Chem. Phys. Letters s5 (1982) 21. [ 151 J.R. Barker, M.J. Rossi and J.R. Pladziewicz, Chem. Phys. Letters 90 (1982) 99. [ 16 1M.J. Rossi, J.R. Pladziewicz and J.R. Barker, J. Chem. Phys. 78 (1983) 6695. [ 171 J.R. Barker, J. Phys. Chem. 88 (1984) 11. [ 181 J.R. Barker and R.E. Golden, J. Phys. Chem. 88 (1984) 1012. [ 191 R.E. Smalley, in: Advances in laser spectroscopy, Vol. 2, eds. B.A. Garctz and J.R. Lombardi (Wiley, New York, 1983) p. 135. PC D.J. Moll, G.R. Parker Jr. and A. Kuppermann, J. Chem. Phys. 80 (1984) 4808. [21 A.S. Sudbo, P.A. Schulz, D.J. Krajnovich, Y.T. Lee and Y.R. Shen, Opt. Letters 4 (1979) 2 19. 122 P.M. Bonell, H.-G. Liihmannsriiben and K. Luther, Ber. Bunsenges. Physik. Chem. 89 (1985) 274. r23 S. Murata, C. Iwanaga, T. Toda and H. Kokubun, Chem. Phys. Letters 13 (1972) 101; 15 (1972) 152; Ber. Bunsenges. Physik. Chem. 76 (1972) 1176. [24] T.M. Woudenberg, S.K. Kulkami and J.E. Kenny, Abstract No. 12 1 (Phys.), 193rd ACS National Meeting, Chicago (April, 1987). [25 ] Y. Hirata and E.C. Lim, J. Chem. Phys. 69 (1978) 3292. [26] M. Comtet and H.D. Mettee, Mol. Photochem. 2 (1970) 63. [27] A.E.W. Knight and B. Selinger, Chem. Phys. Letters 12 (1971) 419. [28] D.R. Demmer, J.W. Hager, G.W. Leach and S.C. Wallace, Chem. Phys. Letters 136 (1987) 329. [29] D. Bouler, W.S. Felps, J. Lewis, R.V. Nauman and S.P. McGlynn, Chem. Phys. Letters 67 (1979) 420. [30] D. Dougherty, J. Lewis, R.V. Nauman and S.P. McGlynn, J. Electron Spectry. 19 ( 1980) 2 1. [3l] R.C. Jamagin, S.-S. Choi and S.S. Takeda, in: Creation and detection of the excited state, Vol. 3, ed. W.R. Ware (Dekker, New York, 1974) p. 134. [32] B.B. Rossi and H.H. Staub, Ionization chambers and counters; experimental techniques (McGraw-Hill, New York, 1949). [33] J.R. Ackerha1tandB.W. Shore, Phys. Rev.Al6 (1977) 277. [34] D.H. Parker, J.O. Berg and M.A. El-Sayed, in: Springer series in chemical physics, Vol. 3. Advances in laser chemistry, ed. A.H. Zewail (Springer, Berlin, 1978) p. 320. [ 351 P.M. Johnson, Accounts Chem. Res. 13 (1980) 20. [36] D.M. Lubman, R. Naaman and R. Zare, J. Chem. Phys. 72 ( 1980) 3034.
CHEMICAL PHYSICS LETTERS
4 March 1988
PHOTOIONIZATION OF HIGHLY VIBRATIONALLY EXCITED GROUND ELECTRONIC STATE AZULENE NEAR THE IONIZATION THRESHOLD
Joseph E. SABOL and Robert W. CARR Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, USA
Received 3 November 1987; in final form 28 December 1987
Highly vibrationally excited ground electronic state azulene, SE, was formed at N, laser excitation of SZat 337,1 nm followed by internal conversion, and photoionized by a time-delayed pulsed dye laser. Ion currents were measured with a parallel plate ion chamber. Photoionization cross sections were estimated by using rate equations to describe ion pmduction rates. The cross sections are 3 X 1O-22cm’ for photoionization by 30303 cm-’ photons, and 6x lO-Z’cm2 by 31153 cm-’ photons.
1. Introduction
Spectroscopic detection of highly vibrationally excited polyatomic molecules presents special problems. For species with vibrational excitation in the range of chemical bond energies, the vibrational state density is usually exceedingly large. Consequently, electronic spectra consist of broad, diffuse bands, and vibrational spectra are poorly understood. Furthermore, lifetimes due to unimolecular reaction and collisional deactivation are typically in the microsecond to nanosecond or even subnanosecond range. Pulsed laser excitation has been used to produce sufficiently large transient populations of highly vibrationally excited ground electronic state molecules to permit direct detection in recent years. Since the broadened and red-shifted (with respect to cold molecules) UV absorption spectra of photoactivated methyl- and ethyl-cycloheptatriene were first reported [ 11, UV absorption spectroscopy has been used to further investigate the cycloheptatrienes [ 2-41, as well as vibrationally hot toluene [ 5-71, CF31 [ 81, benzene [9] and hexafluorobenzene [ 10-121. Detection of photoactivated azulene via infrared emission arising from upper vibrational levels has been employed by Barker and co-workers to follow excited state dynamics [ 13- 181. Although much of the research on multiphoton ionization has been oriented toward investigations of the spectra of both resonant intermediate states
and of ions, the determination of ionization threshold energies, and in combination with mass spectrometric detection to study photoionization and fragmentation mechanisms, it can also be used as a probe to investigate excited state kinetics. Two-color experiments in which the photoionization probe pulse is time delayed with respect to the pump pulse have been used to monitor excited state dynamics. This technique has been successfully used to follow the decay rates of electronically excited molecules [ 19,201. Detection of vibrationally excited ground electronic states has also been reported [21,22]. In two-color pulsed laser photoionization of methylcycloheptatriene with variable time delay between the pulses, Borrell, Lijhmansroben, and Luther [ 221 observed a time-dependent photoion current which was attributed to hot photoproducts that were at least partially relaxed. We report here two-color pulsed laser photoionization of azulene. Highly vibrationally excited ground electronic state azulene (S$) formed by internal conversion of SZ azulene after 337.1 nm excitation, is photoionized by 330 or 321 nm photons which are delayed by a time interval long enough for population of the ground state to be virtually complete, but short enough that it does not, on average, collide before photoionization. The two-photon energy is just greater than the ionization threshold. Photoionization cross sections are small, presumably due to unfavorable Franck-Condon factors.
0 009-26141881%03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
401
Volume 144, number 4
CHEMICAL PHYSICS LETTERS
Azulene is an excellent source of vibrationally hot ground state molecules. The second excited state, which can be conveniently pumped with an Nz laser at 337.1 nm undergoes rapid and efficient internal conversion to the ground state. Fluorescence occurs only to So, with a quantum yield, &=0.03 1 in cyclohexane [ 231, and Woudenberg et al. have concluded from experiments with jet-cooled azulene that intersystem crossing is absent in azulene vapor [ 241. Internal conversion to So occurs both directly and via S,, the former path increasing in importance with increasing excitation energy [25]. Also, the quantum yield for photoisomerization to naphthalene (313nm,0.01 Torr, 300K)is0.5~10-5 [26].Thus, in azulene vapor, the quantum yield for SE formation when excited via SZ is about 0.97. The S2 fluorescence lifetime at 337.1 nm from nanosecond flashlamp excitation has been reported to be 2.09 ns [27]. Recent measurements of the energy dependence of rf( S,) by Demmer et al. [ 281 following picosecond excitation indicate that the lifetime is significantly longer than this. They reported r,( 340 nm) ~3.2 ns and zf( 332 nm) =2.6 ns.
2. Experimental The photoionization apparatus is shown in fig. 1. The ionization chamber is a stainless steel cylinder, 64 mm diameter% 127 mm, fitted with a 25 mm diameter quartz window at each end and two ceramic high vacuum electrical feedthroughs, arranged diametrically at the center of the cylinder wall. A small detachable sample reservoir could be isolated by the
Fig. 1, Photoionization experiment showing excitation/ionization lasers, optical delay path, photoionization chamber, and detection electrmrks. 402
4 March 1988
use of a high vacuum bellows valve. The ion chamber was attached through another bellows valve to a conventional borosilicate 7740 glass vacuum manifold equipped with a Hg diffusion pump, McLeod gauge, and 10d3 to 10 Torr pressure transducer accurate to 0.1 mTorr. A vacuum of 10m6Torr could be maintained on a regular basis. A 28 mmx 115 mm stainless steel repeller plate was connected through one of the ceramic feedthroughs and biased at -45 V to ground, at which voltage the ion current was determined to be independent of the applied field. A stainless steel collector plate, 12.5 mm x 100 mm, was connected to the other ceramic feedthrough and surrounded by a 17.5 mmx 105 mm (inside) by 28 mmx 115 mm (outside) guard electrode which was spot welded to the chamber wall and held at ground potential. This arrangement of the guard electrode ensured a uniform electric field perpendicular to the collector electrode. The separation between the repeller and collector plates was 42 mm. The collector plate was connected by a short length of RG-58/U coaxial cable to the input of a low-noise LHO062 transimpedance preamplifier, gain 10’V/A. The preamplifier output could be observed directly, or amplified 100x by a voltage follower, with the resulting total gain being 10’V/A. The RC time constant of the feedback loop was adjusted so the circuit could reproduce a calibration square wave of 1 ps risetime. Extensive care was taken in the physical layout of the ion chamber/detector/preamplifier in order to minimize the input capacitance. A Faraday cage consisting of two layers of fine mesh brass screen was built around the ion chamber/detector/preamplifier to shield out any RF1 broadcast by the high voltage laser trigger circuit. Single shot waveforms of the amplified ion current were observed and photographed from an oscilloscope, while signal averaging was accomplished by the use of a boxcar averager. The excitation source was a N2 laser producing 10 mJ pulses of 7 ns fwhm duration. A portion of this beam was split off with a quartz microscope slide and passed through a 1 cm diameter aperture prior to entering the cell. High speed silicon photodiodes were used to trigger the electronics form the Nz laser pulse and to monitor the relative pulse to pulse intensity. Typical pulse energy, measured with a pyroelectric
Volume 144, number 4
4 March 1988
CHEMICAL PHYSICS LETTERS
energy/power meter, was 50 l.tJ. The remainder of the nitrogen beam was used to pump a tunable rhodamine 640 dye laser which had a bandwidth of 0.01 nm. Frequency doubling 2v of the dye fundamental v was accomplished with an angle-tuned KDP crystal. The doubled dye pulse energy was typically 10 pJ. The unwanted portion v of the v+2v beam was removed by use of an UV passing filter or a 0.4 m monochromator. The 2v beam entered the photoionization chamber from the end opposite the nitrogen pump beam entrance. An optical delay path separated the pump and photoionization pulses by 20 ns. The pulse separation was verified with a fast silicon photodiode, observing the resulting waveform with the aid of a 2 ns resolution transient digitizer. Azulene was obtained from Aldrich Chemical Company and was purified by repeatedly pumping away the vapor. All experiments reported here were done at a pressure of 20 mTorr of azulene.
L 0
3. Results
The vertical ionization energy of azulene is 7.41 eV [ 29,301. Thus, when excited into SZ at 29665 cm-’ with the N, laser, azulene requires a minimum of 30 110 cm- ’ additional energy to reach the ionization threshold. To ensure that the total energy was above the threshold, the dye laser was tuned to produce a frequency-doubled pulse of either 30303, or 31153 cm-’ in two-color photoionization experiments. Typical ion current waveforms are shown in fig. 2. Photoionization was observed in onscolor photoionization with the N2 laser, fig. 2a. The energy of two 29665 cm-’ photons is 7.35 eV, which is approximately 0.05 eV below the ionization threshold, Ionization caused by three or more photons is most probably not the cause of this signal since a study of the total charge produced, q, as a function of NZ laser intensity I showed that the slope of a plot of log q versus log I is 2.OkO.1, and the rate of the So-S2 transition is not expected to be much faster than transitions from Sz and higher states. The total number of positive charges produced is obtained by integrating the current waveform,
50
Time, Microsecond Fig. 2. (a) Photoion current from two-photon, one-color (337.1 nm) excitation. The total number of ions produced is obtained from the integrated photoion current, q=Ji dt. The area shown herecorrespondsto (1.8+0.2)~10’ionsor ~3xlO-‘~C. (b) Detector/amplifier output from two-photon, one-color (330.0 nm) excitation, essentially base line. (c) Photoion current from two-photon, two-color (337.1 nm + 330.0 nm, 20 ns delay) excitation. Note the signal from the two-photon, one-color (a) excitation is still present, and the total photoion current is enhanced by the 330.0 nm ionization of St.
CQ
4=
s
idt.
0
Thus, photoionization must be aided by thermal energy. At room temperature 14% of the azulene molecules have more than 0.05 eV of thermal energy which is sufficient, with the addition of two 29665 cm-l photons, to take them to the ionization threshold. The waveforms displayed in fig. 2 are due to transport of azulene ions to the electrodes. The ion transit time can be estimated by the equation t=L21mV, 403
Volume 144, number 4
CHEMICALPHYSICS LETTERS
where L is the interelectrode spacing, m is the ion mobility and Vis the applied voltage [ 3 11. Since no report of the azulene ion mobility was found in the literature the value 0.2 cm2/V s at 1 atm was estimated from data for other species [ 321. Ion mobility scales inversely with pressure. A pressure of 0.02 Torr gives a mobility of 7.6 x lo3 cm2/V s and a transit time of 5.2 X lOa s. This is remarkably good agreement with the 50 its times observed. The electron transit time was estimated to be approximately 1O-’ s, too fast for the microsecond risetime of the detection circuitry, and waveforms due to electrons were not observed. Due to the relatively slow transport of ions, these ion currents contain no information on the much faster azulene photophysics. The positive ion yield obtained by integrating the photoion current waveform gives the total photoion yield from all photoionization paths. Fig. 2b shows that no photoion current was detected when azulene was exposed to only the dye laser (N2 laser beam blocked) while f’ig.2c shows results with the N2 laser and the 20 ns delayed pulse from the dye laser. Laser operating conditions were held constant during each set of experiments, figs. 2a-2c. The current waveform was repetitively scanned by the boxcar to average pulse to pulse variations in laser energy. The absorption of two 30303 cm-’ photons, fig. 2b, from the dye laser result in azulene with 7.5 1 eV, well above the ionization threshold. The combination of modest dye laser energy (10 pJ) and low absorption by azulene at this wavelength produces ion currents too small for detection with the present ion chamber/detector/preamplifier. Fig. 2c shows the current waveform resulting from excitation by the 29665 cm-’ pulse followed (separated in time but overlapped in space) by the 30303 cm-’ pulse, total energy 7.43 eV. The amplitude of the waveform in fig. 2a is clearly greater than the amplitude shown in fig, 2a. Furthermore, integration of the curves revealed that the number of ions produced using both lasers is larger than the number formed from the N2 laser alone. Since the enhanced signals in the twocolor time-delayed experiment are only slightly greater than the sum of the one-color ion currents from figs. 2a and 2b, the experiment was repeated several times with the dye laser at 30303 cm-‘, and also at 31153 cm-‘. These data are summarized in 404
4 March 1988
Table 1 Number of ions produced under various photoionization conditions Excitation (cm-‘)
Number of ions
Charge (C)
29665 30303 29665+30303 enhancement (29665+30303)-29665 31153 29665+31153 enhancement (29665+31153)-31153
1.8~10~
2.9x lo-l4
2.0x 105
3.2x lo-l4
0.2~10~ 4.6~10~ 8.2~10’
0.3x lo-l4 7.4x lo-l4 1.3x 10-I’
3.6x10’
5.9x 10-14
table 1. In the case of the one-color two-photon 3 1153 cm-’ excitation, azulene containing 7.72 eV is formed and substantial ion current was observed. Nevertheless, the ion signal is enhanced when the N2 laser pulse precedes the dye laser pulse.
4. Discussion The one-color photoionization of azulene by the N2 laser is a resonance-enhanced two-photon ionization via SZ. In the two-color experiments this signal is still present, and the additional ions created are due to photoionization induced by the dye laser. At 20 mTorr total pressure, the collision interval is significantly longer than the delay time between the laser pulses, and the experiment can safely be considered to occur in the absence of collisions. When the dye laser pulse arrives, the SZpopulation has decayed to less than 0.1% of its peak value. Thus, the dye laser interrogates an excited state population dominated by St with 29665 cm-’ of vibrational energy. Experiments done with pure naphthalene (S, origin 32018 cm-‘, IP 8.13 eV) samples at pressures of approximately 100 mTorr gave photoion currents approximately 60% smaller than the azulene signals. Since naphthalene yields from photoisomerization of azulene are expected to be small ( Q: 1 mTorr), the contribution of the ion signals from naphthalene is negligible. An estimate of the cross section for photoionization of S$ was made by obtaining an approximate solution to the set of coupled differential equations describing the applicable azulene excited state ki-
CHEMICAL PHYSICS LETTERS
Volume 144. number 4
netics. The combination of small laser pulse energy, and optically thin samples provide appropriate conditions for the application of rate equations [ 33,341. Azulene photoionization can be described by the following mechanism: hv(29665 cm-‘) +S0+S2,
~=~02~,(0&,
SZ-&+hv’,
r=kfS, >
s24I:
r=kS2
3
hv(29665 cm-‘)+!$+A:
+e-,
,
T=[TZiZi(f)SZ,
4 March 1988
as confirming the weak excitation condition for using rate equations. The rate of ion formation in the one-color photoionization is ti:ldt=oziZi&(t)f,
(3)
wherefis the fraction of S2 azulenes with enough energy to be photoionized with another N2 laser photon. At 300 Kf= 0.145. Since ions are only produced when the laser is on, S2( t) can be expressed by eq. (2). Integrating eq. (3) using the initial condition A: =O at t=O gives
hv(30303 cm-‘) +S$-+A,+ +e- , r=ooiZ2(t)Sg, The (T’Sare photoabsorption cross sections, Z,(t) and Z2(t) are the time-dependent intensities (photons/cm* s) of the N2 laser and dye laser, respectively, A+ is the azulene photoion, e- the photoelectron, and r the rate per unit volume. The rate of formation of S2 is given by dS,ldt=o,,Z,(t)S,,-
[~+k,,+a2iZ,(t)]S2
.
(1)
In eq. (1) , CziZ,(t) can be neglected as it turns out to be about three orders of magnitude smaller than k, + k,,. The value of Z++kIc= l/t used here is 3.1 x lo8 s- ‘, interpolated from data on the energy dependence of the S, lifetime reported in ref. [ 281. Also, uo2= 5.4x10-18cm2wasused[15].Intheintegration of eq. (1) and other differential equations below a square wave approximation to both I, (t) and Z2(t) is used. While this is an oversimplification of the actual laser pulse shape, it has the virtue that analytical solutions are obtained, while not introducing large errors into the calculated photoionization cross sections. The solution of (1) for the initial condition S,(O) =O is S2(t)=doZZ,S07[1-exp(
-t/z)]
,
(2)
where II = 0 at t < 0 and t > 10 ns; I, = constant in the range 0 < t6 10 ns. Thus eq. (2) describes S2( t) only during the interval during which the pulse occurs. Thereafter S,(t) is described by an exponential decay, S2(t)=S2(t=10 ns) exp[-(t-10)/r], where S2( t = 10 ns) is calculated by eq. (2). The steady state value of S,, obtained when Z,(t) is a step function, is given as t-m by S,,= c~~~Z,S,,~. At 10 ns, S2( t) has risen to 95% of S,,. Furthermore, S2JS0= 3 x 10-3, which justifies S,= constant in integrating (1) , as well
A: =~O2U2iZ~SOfZ{t-r[l-exp(-t/r)]}.
(4)
Using measured values of A : with the N2 laser alone (table 1 ), eq. (4) can be used to calculate g2i, the cross section for photoionization of Sz. For t = 10 ns, 62i= 1.7X 10W20cm2. The cross section for photoionization of Sz can be obtained by considering the rate of production of ions from Sz. The ion signal, A:, in the two-color photoionization corresponds to the enhancement (table l), dA,fldt=o,,Z,(t)S;:.
(5)
Since S2( t) has virtually completely decayed by the time the dye laser pulse arrives, S$ will be stationary in the absence of collisions and other removal paths, and can be regarded as a constant in the integration of (5). Furthermore, Sz will correspond to 97% of the ground state depletion, -dSoldt=ao2L,(t)So.
(6)
Integrating (6) over the 10 ns N2 laser pulse, AS,,=3,78x 10” cm-3. Letting St=0.97ASo and integrating (5) over the 7 ns dye laser pulse gives AZ =ooiZ2(t)S$Ats Using the enhancement of ion current from table 1, we calculate hoi= 3.3 x 10TL2cm2 with the dye laser tuned to 30303 cm-‘, and aoi=6.0XlO-*’ cmm2 with the dye laser tuned to 3 1153 cm- ‘. While these cross sections must be regarded as approximate, the uncertainties introduced by idealization of the laser pulse shape are probably not large, and the calculations are expected to reveal the correct magnitudes of the cross sections. The very small cross sections for ionization of Sz are not unex405
Volume 144, number 4
CHEMICAL PHYSICS LETTERS
petted since the Franck-Condon factors for excitation of SE, which contains approximately 90 kcal/mol of vibrational energy, to the vibrationally cold ion are expected to be very unfavorable [ 351. Also, the relatively small cross section for ionization of SZ, w 10V20cm2, may be indicative of bond length differences between the SZstate of azulene and the molecular ion. Lubman et al., in their study of azulene photoionization, found no ionization in two-color experiments with the beams spatially overlapped but timedelayed [ 361. This is not in conflict with the results reported here since: (1) the excitation mechanism was different and (2) the detector (mass spectrometer-channeltron) used probably has significantly less absolute sensitivity than our ion chamber.
Acknowledgement Acknowledgement is made to the Donors of the Petroleum Research Fund, administered by the American Chemical Society, for the support of this research.
References [ I] H. Hippler, K. Luther, J. Troe and R. Walsh, J. Chem. Phys. 68 (1978) 323. [2] H. Hippler, K. Luther and J. Troe, Faraday Discussions Chem. Sot. 67 (1979) 173. [ 31 H. Hippler, J. Troe and H.J. Wendelken, J. Chem. Phys. 78 (1983) 6718. [ 41 II. Hippler, K. Luther, J. Troe and H.J. Wendelken, J. Chem. Phys.79 (1983) 239. [ 51H. Hippler, J. Troe and H.J. Wendelken, Chem. Phys..Letters84 (1981) 257. [ 61 H. Hippler, V. Schubert, J. Troe and H.J. Wendelken, Chem. Phys. Letters 84 (1981) 253. [ 71 H. Hippler, J. Troe and H.J. Wendelken, J. Chem. Phys. 78 (1983) 6709. [g] Yu.A. Kudriatsev and V.S. Letokhov, Chem. Phys. 50 (1980) 353. (91 N, Nakashima and K. Yoshihara, J. Chem. Phys. 79 (1983) 2727. [lo] T. Ichimura, Y. Mori, N. Nakashima and K. Yoshihara, Chem. Phys. Letters 104 (1984) 533.
406
4 March 1988
[ 111 T. Ichimura, Y. Mori, N. Nakashima and K. Yoshihara, J. Chem. Phys. 83 (1985) 117. [ 121 T. Ichimura, M. Takahashi and Y. Mori, Chem. Phys. I 14 (1987) 111. [ 13 ] G.P. Smith and J.R. Barker, Chem. Phys. Letters 78 (198 1) 253. [ 141 M.J. Rosi and J.R. Barker, Chem. Phys. Letters s5 (1982) 21. [ 151 J.R. Barker, M.J. Rossi and J.R. Pladziewicz, Chem. Phys. Letters 90 (1982) 99. [ 16 1M.J. Rossi, J.R. Pladziewicz and J.R. Barker, J. Chem. Phys. 78 (1983) 6695. [ 171 J.R. Barker, J. Phys. Chem. 88 (1984) 11. [ 181 J.R. Barker and R.E. Golden, J. Phys. Chem. 88 (1984) 1012. [ 191 R.E. Smalley, in: Advances in laser spectroscopy, Vol. 2, eds. B.A. Garctz and J.R. Lombardi (Wiley, New York, 1983) p. 135. PC D.J. Moll, G.R. Parker Jr. and A. Kuppermann, J. Chem. Phys. 80 (1984) 4808. [21 A.S. Sudbo, P.A. Schulz, D.J. Krajnovich, Y.T. Lee and Y.R. Shen, Opt. Letters 4 (1979) 2 19. 122 P.M. Bonell, H.-G. Liihmannsriiben and K. Luther, Ber. Bunsenges. Physik. Chem. 89 (1985) 274. r23 S. Murata, C. Iwanaga, T. Toda and H. Kokubun, Chem. Phys. Letters 13 (1972) 101; 15 (1972) 152; Ber. Bunsenges. Physik. Chem. 76 (1972) 1176. [24] T.M. Woudenberg, S.K. Kulkami and J.E. Kenny, Abstract No. 12 1 (Phys.), 193rd ACS National Meeting, Chicago (April, 1987). [25 ] Y. Hirata and E.C. Lim, J. Chem. Phys. 69 (1978) 3292. [26] M. Comtet and H.D. Mettee, Mol. Photochem. 2 (1970) 63. [27] A.E.W. Knight and B. Selinger, Chem. Phys. Letters 12 (1971) 419. [28] D.R. Demmer, J.W. Hager, G.W. Leach and S.C. Wallace, Chem. Phys. Letters 136 (1987) 329. [29] D. Bouler, W.S. Felps, J. Lewis, R.V. Nauman and S.P. McGlynn, Chem. Phys. Letters 67 (1979) 420. [30] D. Dougherty, J. Lewis, R.V. Nauman and S.P. McGlynn, J. Electron Spectry. 19 ( 1980) 2 1. [3l] R.C. Jamagin, S.-S. Choi and S.S. Takeda, in: Creation and detection of the excited state, Vol. 3, ed. W.R. Ware (Dekker, New York, 1974) p. 134. [32] B.B. Rossi and H.H. Staub, Ionization chambers and counters; experimental techniques (McGraw-Hill, New York, 1949). [33] J.R. Ackerha1tandB.W. Shore, Phys. Rev.Al6 (1977) 277. [34] D.H. Parker, J.O. Berg and M.A. El-Sayed, in: Springer series in chemical physics, Vol. 3. Advances in laser chemistry, ed. A.H. Zewail (Springer, Berlin, 1978) p. 320. [ 351 P.M. Johnson, Accounts Chem. Res. 13 (1980) 20. [36] D.M. Lubman, R. Naaman and R. Zare, J. Chem. Phys. 72 ( 1980) 3034.