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Coherent infrared–ultraviolet double-resonance spectroscopy of CH3
Chemical Physics Letters 370 (2003) 204–210 www.elsevier.com/locate/cplett
Coherent infrared–ultraviolet double-resonance spectroscopy of CH3 Thomas B. Settersten a
a,*
, Roger L. Farrow a, Jeffrey A. Gray
b
Combustion Research Facility, Sandia National Laboratories, P.O. Box 969, MS 9056, Livermore, CA 94551, USA b Department of Chemistry, Ohio Northern University, Ada, OH 45810, USA Received 25 November 2002; in final form 7 January 2003
Abstract Two-color polarization spectroscopy (TC-PS) and two-color resonant four-wave mixing spectroscopy (TC-RFWM) are used to detect photolytically produced CH3 radicals. An infrared laser pumps individual lines in the m3 fundamental ~ 2 A00 state, and an ultraviolet laser probes the pumped levels to reveal rotationally resolved spectra of transitions of the X 2 ~ 2 A0 state. The spectra are fit with a complex Lorentzian lineshape and yield an updated value of to the predissociated B 1 1 ~ state. A detection limit of 2 1013 CH3 molecules per cm3 per quantum state is 46 239:4 1:2 cm for T0 of the B observed for these coherent double-resonance techniques. Ó 2003 Elsevier Science B.V. All rights reserved.
1. Introduction Free methyl radical (CH3 ) is a primary intermediate in flames, plasmas, and photolytic reactions and is also observed in planetary atmospheres and interstellar clouds. Because of its chemical importance and structural simplicity, CH3 has been the subject of many spectroscopic studies. The development of optical techniques for the quantitative detection of CH3 remains an important research focus. Ultraviolet (UV) laser-induced fluorescence detection of CH3 is not possible because predissociation of its electronically excited states is rapid, and experimentalists have resorted to
*
Corresponding author. Fax: +1-925-294-2595. E-mail address: [email protected] (T.B. Settersten).
absorption-based techniques for optical detection of CH3 . Direct absorption in either the m3 vibrational fundamental in the mid-infrared [1–3] (IR) ~ 2 A0 ~ 2 A00 electronic system in the ulor the B X 1 2 traviolet [4–7] (UV) have been used for quantitative measurement of path-integrated CH3 concentration. Non-linear optical techniques such as REMPI [8,9], CARS [10], and DFWM [11,12] have been used for spatially resolved CH3 measurements. Spectral interferences in both the IR and UV regions, however, can cause large systematic uncertainties for all of these techniques. Although the ~ ~ origin band near 216 nm has predissociated B X a large peak cross-section, it lacks rotational structure, which complicates accurate simulation of the band contour under different conditions. In recent experiments, we demonstrated the viability of IR–UV double-resonance spectroscopy
0009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0009-2614(03)00062-9
T.B. Settersten et al. / Chemical Physics Letters 370 (2003) 204–210
for spatially resolved detection of the hydroxyl radical in complex reacting flows [13]. In the present work, we use the coherent versions of this technique, two-color polarization spectroscopy (TC-PS) and two-color resonant four-wave mixing (TC-RFWM) spectroscopy, for the detection of CH3 in moderate-to-high concentrations. The imposition of a double-resonance condition offers enhanced species selectivity and immunity to systematic biasing due to spectral interferences. We observe the first fully rotationally resolved spectra ~ –X ~ system of CH3 , which provide a precise of the B value for the vibronic origin and give direct evidence that the predissociation rate does not depend significantly on orientation of the rotating molecules.
2. Experiments The relevant energy level diagram for the double-resonance experiments is shown in Fig. 1 with the pump, probe, and signal transitions indicated by arrows. We label rotational quantum numbers ~ 2 A00 groundin the m3 ¼ 0 and m3 ¼ 1 levels of the X 2 00 00 0 0 state of CH3 as (N ; K ) and (N ; K ), respectively. IR transitions between these levels obey perpendicular selection rules DN ¼ 0 (K 6¼ 0), 1 and DK ¼ 1. Using the molecular constants determined by Amano et al. [1], our calculated IR transition frequencies agreed with all of our measured transition frequencies of the fully resolved
Fig. 1. Energy level diagram for IR–UV double resonance spectroscopy of CH3 .
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DNK00 ðN00 Þ lines to within the pump laser bandwidth. The probe laser was tuned to transitions in ~ ~ origin band. We label rotational levels the B X 2 0 ~ in the B A1 Rydberg (3s) state of CH3 as (N ; K ) and denote probe transitions as DNK00 ðN00 Þ. The UV transitions obey parallel selection rules DN ¼ 0 (K 6¼ 0), 1 and DK ¼ 0. All of the double-resonance experiments described here involved transitions having common (N00 ; K00 ) levels as shown in Fig. 1. Methyl radicals were produced by flash photolysis of acetone or CH3 I using up to 30 mJ of the fourth-harmonic output of a 6-ns Q-switched Nd:YAG laser. All experiments were conducted at room temperature. For experiments at atmospheric pressure, a flow of argon (200 sccm) bubbled through acetone at 7°C (v.p. ¼ 100 Torr) and passed into the laboratory through a 50-mm diameter honeycomb that was used to stabilize the flow. The photolysis laser was formed into a horizontal sheet (1 40 mm) and directed through the gas flow. Production of CH3 was accompanied by loud acoustic bursts. For experiments at pressures below 1 Torr, CH3 I vapor was drawn through a 7-cm steel vacuum cube. Fused silica windows were mounted directly on two sides of the vacuum cube to allow the photolysis laser sheet (1 10 mm) to pass through the cell. Two 30-cm extensions were attached to other sides of the cube and capped with fused silica windows to allow the pump, probe, and signal beams to enter and exit the cell in a direction nominally perpendicular to the photolysis laser. The pump and probe lasers used in these experiments are similar to those described previously for double-resonance detection of OH [13]. The second harmonic output of a 1.5-ns pulsed Nd:YAG laser pumped two dye lasers. An IR pump beam tunable near 3.1 lm was generated by difference-frequency mixing the output of one of the dye lasers with the 532-nm output of the Nd:YAG laser. A UV probe beam tunable near 216 nm was generated by frequency tripling the output of the other dye laser. The pump and probe pulse energies each were monitored during the experiments using pyroelectric joule meters. The pump laser produced pulse energies of up to 500 lJ, while the probe laser produced pulse DK
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energies of up to 100 lJ. The wavenumber of the fundamental output from each dye laser was monitored and calibrated to within 0.1 cm1 . The pump and probe beams were crossed in the sample with respective beam diameters of approximately 1.5 and 1 mm. The optical paths were arranged so that the probe pulses reached the sample 0.25 ns after the pump pulses. For most experiments, the pump and probe laser pulses were delayed by 200 ns with respect to the photolysis pulse to allow for some thermalization. TC-PS experiments were performed with the pump beam polarized vertically and the probe beam polarized linearly at 45°. The beams crossed through the photolysis region at a full angle c ¼ 2° (Fig. 2a) and overlapped for approximately 4 cm. An a-BBO prism polarizer was placed in the UV beam after the sample and rotated to null the beam with an extinction ratio of 5 108 at 216 nm. The transmitted UV beam was focused through a 300lm diameter pinhole to reduce scattered light and then detected through UV-bandpass and neutraldensity filters by a UV-sensitive photomultiplier tube (PMT). The PMT output was integrated with a charge-integrating amplifier, digitized, and normalized by the probe pulse energy. Typically 100–500 laser shots were averaged for each measurement. TC-PS was not used to detect the CH3 produced by the photolysis of CH3 I because the vacuum cell windows severely degraded the polarization. TC-RFWM experiments were performed with all beams polarized vertically and propagating
Fig. 2. Experimental arrangement for (a) TC-PS and (b) TCRFWM.
forward in the same plane (Fig. 2b). The pump beam was split into two approximately equal-intensity beams that crossed at a full angle b ¼ 6:3°, resulting in an overlap of approximately 3 cm in the sample. Phase matching was achieved with the probe and signal beams emerging separated by 0.44°. For this geometry, the phase-matching bandwidth was more than sufficient to include all linked sets of spectral transitions [14]. Signals were detected as with TC-PS except that a 25-lm diameter pinhole was used to suppress scattered light. Additionally, several adjustable apertures were placed in the probe and signal beams to minimize this background.
3. Results and discussion 3.1. UV double-resonance spectra The first set of double-resonance experiments investigated TC-RFWM probe spectra with the pump laser tuned to particular IR transitions. Pump transitions in the r R3 or r R6 branches were strongest because the H€ onl–London factors are largest for the r R branches [15], and levels with K00 ¼ 3q (q is an integer) have double the statistical weight of the other K00 levels due to nuclear spin statistics. While double-resonance probe spectra at 1 atm appear nearly as broad as the entire absorption band, at reduced pressures collisional redistribution between (N00 ; K00 ) levels during the pump/probe interaction is negligible, and TCRFWM probe spectra observed at 1 Torr (Fig. 3) are well resolved into one set of P, Q, and R lines. The probe spectra are described by a line shape function LðmÞ that is equal to the modulus squared of a complex Lorentzian function: X A Dm 2 X A C 2 i i i i LðmÞ ¼ ð1Þ þ ; i Dm2i þ C2i i Dm2i þ C2i where Ai is the amplitude, Ci is the linewidth (HWHM), and Dmi is the detuning from each probe resonance i. The line shape function was derived using the theoretical treatment of Williams et al. [16]. For the TC-RFWM scheme employed here (Scheme 3 with YYYY polarization configu-
T.B. Settersten et al. / Chemical Physics Letters 370 (2003) 204–210
~ ~ origin Fig. 3. TC-RFWM probe spectra of the CH3 B X band at 1 Torr with the pump laser fixed on the r R3 (4) (bottom), r R3 (5) (middle), and r R3 (6) (top) transitions. Experimental data are shown as points. The solid curves represent the best-fits to the data using the line shape function given by Eq. (1).
ration in [16]), the signal results from ground-state gratings as well as from coherences between the excited states of the pump and probe transitions. In the present experiments, however, coherence ~ dephasing is dominated by predissociation in the B state [12], and the coherences between the excited states contribute negligibly to the RFWM signal because they are rapidly damped. The result is a single resonant denominator that corresponds to the ground-state grating. Eq. (1) accounts for interference occurring between multiple probe transitions. Inclusion of this complicating effect, which is also observed in other coherent techniques such as CARS, is essential to obtain meaningful fit parameters. We recorded and fitted with Eq. (1) probe spectra involving more than 20 different double resonances with the pump laser fixed on various isolated lines. The relative magnitudes of the P, Q, and R lines in each TC-RFWM spectrum (Ai in Eq. (1)) were constrained with calculated line
207
strengths [16]. The best-fit values for the frequencies and linewidths of the probe transitions are listed in Table 1. The precision of our measured ~ –X ~ rotational transitions is line positions for B limited by their large linewidths, and thus we cannot improve upon the established bond length ~ state that was inferred from structure of for the B ~ –X ~ origin band of CD3 [4,17]. By completely the B resolving these transitions for the first time, however, we can precisely determine the vibronic origin as T0 ¼ 46 239:4 1:2 cm1 from a weighted least-squares fit of computed line positions to measured line positions using fixed rotational ~ state (B ¼ 2C ¼ 8:827 constants [17] for the B cm1 ). Deviations of the fitted line positions from the computed positions are listed in Table 1. We estimate that a non-resonant background in our TC-RFWM spectra could cause an additional systematic uncertainty of up to 5 cm1 in our reported T0 value. Our result is compared with previous reports of T0 in Table 2. HerzbergÕs T0 value [17] represents the center of unresolved Q and P branches observed in absorption following flash photolysis of HgðCH3 Þ2 . More recently Westre et al. [18] determined T0 by fitting rotational resonance Raman excitation profiles. Te and T0 values have also been calculated by Botschwina et al. [19] and Mebel and Lin [20]. Westre et al. [18] measured linewidths for a series of N levels and used them to model the ~ potential barrier for predissociation of the B state. Their model suggests that widths for K ¼ N levels should be 10–20% smaller than widths for K ¼ 0 levels of the same N , although K was not resolved in their measurements. Our K -resolved widths generally reproduce the N
dependence observed by Westre et al. [18] and our precision is sufficient to confirm that the rotational predissociation rate depends weakly on molecular orientation (N ), decreasing <15% with increasing K . At low pressures TC-RFWM signals involving N00 ¼ K00 levels exhibited a short-lived, 20-fold enhancement when the polarization of the photolysis laser was parallel to the (vertical) pump and probe lasers. This observation indicated a preferential orientation of the CH3 photofragments with rotation around the C3 top axis, and is consistent
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Table 1 Observed coherent IR–UV double-resonance transitions for CH3 Pump transition
Wavenumber (cm1 )
Probe transition
Observed wavenumber ð2rÞ (cm1 )
O–Ca (cm1 )
r
3211.5 3211.5 3211.5 3211.5 3209.6 3209.6 3209.6 3199.7 3199.7 3217.7 3217.7 3217.7 3217.7 3217.7 3217.7 3217.7 3217.7 3217.7 3235.5 3235.5 3235.5 3235.5 3235.5 3235.5 3216.2 3282.5 3282.5 3252.9 3252.9 3224.3 3224.3 3224.3 3270.0 3270.0 3232.3 3248.1 3248.1
P0 (2) R0 (2) P0 (2) R0 (2) P2 (3) Q2 (3) R2 (3) Q3 (3) Q3 (3) P3 (4) Q3 (4) R3 (4) P3 (4) Q3 (4) R3 (4) P3 (4) Q3 (4) R3 (4) P3 (5) Q3 (5) R3 (5) P3 (5) Q3 (5) R3 (5) Q5 (5) P0 (6) R0 (6) P3 (6) R3 (6) Q6 (6) Q6 (6) Q6 (6) P3 (7) R3 (7) Q7 (7) Q9 (9) Q9 (9)
46 194.1 (6.3) 46 288.7 (7.4) 46 201.7 (4.4) 46 293.5 (3.3) 46 235.1 (6.1) 46 156.8 (6.4) 46 216.5 (4.1) 46 235.6 (1.4) 46 244.0 (4.2) 46 158.4 (7.6) 46 221.2 (7.4) 46 319.6 (7.3) 46 163.5 (4.6) 46 227.6 (2.7) 46 323.8 (5.2) 46 152.2 (5.2) 46 223.4 (2.5) 46 319.9 (2.9) 46 134.5 (6.7) 46 225.4 (4.9) 46 321.9 (2.5) 46 139.0 (6.7) 46 223.3 (4.9) 46 330.1 (2.5) 46 228.7 (2.9) 46 109.0 (11.5) 46 338.9 (4.8) 46 104.0 (2.6) 46 328.7 (2.8) 46 222.9 (2.7) 46 217.9 (3.7) 46 225.8 (2.7) 46 071.9 (4.6) 46 343.1 (7.9) 46 220.4 (5.0) 46 200. (3.3) 46 197.2 (3.6)
)5.5 0.8 2.1 5.6 9.0 7.7 5.9 1.4 9.8 0.7 )7.1 3.0 5.8 )0.7 7.2 )5.5 )4.9 3.3 1.7 4.3 )5.1 6.2 2.2 3.1 1.2 5.7 6.1 )2.6 )7.4 )0.3 )5.3 2.6 )7.4 )1.0 2.2 )6.4 )9.3
R0 (2) R0 (2) r R0 (2) r R0 (2) r R2 (3) r R2 (3) r R2 (3) r R3 (3) r R3 (3) r R3 (4) r R3 (4) r R3 (4) r R3 (4) r R3 (4) r R3 (4) r R3 (4) r R3 (4) r R3 (4) r R3 (5) r R3 (5) r R3 (5) r R3 (5) r R3 (5) r R3 (5) r R5 (5) r R0 (6) r R0 (6) r R3 (6) r R3 (6) r R6 (6) r R6 (6) r R6 (6) r R3 (7) r R3 (7) r R7 (7) r R9 (9) r R9 (9) r
a
84 (17) 103 (19) 79 (10) 82 (9) 83 (13) 76 (14) 93 (9) 86 (4) 101 (13) 80 (18) 94 (20) 103 (16) 80 (10) 74 (7) 110 (10) 81 (9) 70 (6) 84 (6) 106 (12) 69 (10) 80 (7) 98 (9) 75 (9) 88 (6) 98 (10) 112 (25) 86 (7) 98 (7) 97 (11) 93 (14) 88 (14) 81 (15) 111 (18) 93 (20) 78 (12) 104 (15)
Probe wavenumber calculated from best-fit T0 .
Table 2 B–X vibronic origin values for CH3 T0 ðcm1 Þ
Reference
46 239:4 1:2 46 300 50 46 205 46 027a 46 435
This work [18] [17] [19] [20]
a
FWHM ð2rÞ (cm1 )
Adjusted from Te using calculated zero-point energies [20].
with the dissociation process proceeding in C3v symmetry. In this case, top-axis rotation of the methyl iodide parent molecule results in preferential N00 ¼ K00 orientation of the photofragment [21]. At 0.7 Torr the enhancement was observed to decay in approximately 10 ns. The fast collisional reorientation rate is consistent with large nonthermal velocities of CH3 fragments following photodissociation of CH3 I with 266-nm photons [9].
T.B. Settersten et al. / Chemical Physics Letters 370 (2003) 204–210
3.2. IR double-resonance spectra Pump spectra were recorded with the probe laser fixed at 46 200 cm1 , where most of the Q-branch lines overlap. A TC-PS signal obtained from CH3 at 1 atm is plotted as a function of pump frequency in Fig. 4. This polarization spectrum consists of three isolated CH3 transitions in the same region where lines from stable molecules such as CH4 or H2 O can overlap badly in direct IR-absorption spectra [3]. The relatively large intensity of the r R6 (6) line is expected because of the larger population of the N00 ¼ K00 ¼ 6 level at 293 K and the quadratic dependence of TC-PS signals on molecule density [18]. The observed TC-PS linewidths were consistent with the pump laser bandwidth ( 0.2 cm1 ) and a Lorentzian-squared line shape function, and destructive interference was evident between the closely spaced r R6 (6) and p Q7 (7) lines. We did not analyze fits to the pump spectra, however, because the resonances in the m3 band are already accurately known, and rapid collisional redistribution among quantum states during the pump/probe interaction makes it difficult to absolutely quantify the rotational line intensities. 3.3. Saturation and detection limits Optical saturation of the double-resonance signals was evident at modest laser intensities. The
Fig. 4. Rotational transitions in the m3 fundamental band of CH3 observed using flash photolysis/TC-PS at atmospheric pressure. The solid curve represents a best-fit profile using a Lorentzian-squared line shape function.
209
pump laser saturation irradiance was approximately 5 MW/cm2 for TC-PS at 1 atm, which is similar to the value observed for OH in a 1-atm flame [18], and approximately 0.7 MW/cm2 for TC-RFWM at 1 Torr. The probe laser saturation irradiance was approximately 11 MW/cm2 for TCPS at 1 atm and 0.9 MW/cm2 for TC-RFWM at 1 ~ state is on the Torr. Because the lifetime of the B order of 104 times shorter than the laser pulse, stimulated emission was insignificant, and probe saturation is best understood as depletion of the ground-state population. We observed signal-to-noise ratios that were on the order of 300 for TC-PS at 1 atm and 30 for TCRFWM at 1 Torr, both of which were somewhat smaller than observed for OH in a 1-atm flame [18]. Detection limits were based on measurements of CH3 concentration resulting from direct absorption of the probe beam [11,12,22]. A background-corrected absorption spectrum and the simulated spectrum are shown in Fig. 5. The poor agreement at the R-branch head may indicate that the CH3 did not reach thermal equilibrium with the bath gas. The observed peak absorption corresponds to a molecule density that is on the order of 1016 cm3 , which is consistent with the 1-ls half life for CH3 we observed and the known rate constant for self reaction at 300 K [23]. We estimated that the molecule density resulting from low-pressure photolysis of CH3 I was approximately 30% of that observed when acetone was photolyzed at atmospheric pressure. Based on
Fig. 5. The observed absorption spectrum of CH3 B–X origin band at 1 atm following photolysis of acetone. The solid curve is the band contour simulated at 293 K using our T0 value and a linewidth of 80 cm1 .
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these estimates, TC-PS and TC-RWFM have a detectivity limit of 2 1013 CH3 molecules per cm3 per quantum state. 4. Conclusions We have demonstrated the use of coherent IRUV double-resonance techniques for the detection of CH3 at room temperature. The state specificity afforded by the double-resonance condition resulted in the first fully rotationally resolved UV ~ –X ~ origin band of CH3 , enabling spectra in the B direct spectroscopic characterization of the pre~ state. While these new coherent dissociated B techniques are not nearly as sensitive as CRDS [3,7], they have advantages as diagnostics for complex, reacting flows at higher pressures because of their higher spatial resolution and analyte specificity. We expect that TC-PS and TC-RFWM spectroscopy of CH3 and other non-fluorescing free radicals will be significantly improved using single-mode picosecond lasers. The expected reductions of collisional depolarization [24,25], and shot-to-shot fluctuations of the laser will dramatically improve signal-to-noise ratios. Acknowledgements The authors appreciate the excellent laboratory assistance of Mr. Paul Schrader and support from the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences. References [1] T. Amano, P.F. Bernath, C. Yamada, Y. Endo, E. Hirota, J. Chem. Phys. 77 (1982) 5284. [2] G.A. Bethardy, R.G. Macdonald, J. Chem. Phys. 103 (1995) 2863.
[3] J.J. Scherer, K.W. Aniolek, N.P. Cernansky, D.J. Rakestraw, J. Chem. Phys. 107 (1997) 6196. [4] G. Herzberg, Proc. R. Soc. London Ser. A 262 (1961) 291. [5] D.F. Davidson, A.Y. Chang, M.D. Di Rosa, R.K. Hanson, J. Quant. Spectrosc. Radiat. Transfer 49 (1993) 559. [6] D.F. Davidson, M.D. Di Rosa, E.J. Chang, R.K. Hanson, J. Quant. Spectrosc. Radiat. Transfer 53 (1995) 581. [7] P. Zalicki, Y. Ma, R.N. Zare, E.H. Wahl, J.R. Dadamio, T.G. Owano, C.H. Kruger, Chem. Phys. Lett. 234 (1995) 269. [8] J.W. Hudgens, T.G. Di Giuseppe, M.C. Lin, J. Chem. Phys. 79 (1983) 571. [9] D.H. Parker, Z.W. Wang, M.H.M. Janssen, D.W. Chandler, J. Chem. Phys. 90 (1989) 60. [10] N.E. Triggs, M. Zahedi, J.W. Nibler, P. DeBarber, J.J. Valentini, J. Chem. Phys. 96 (1992) 1822. [11] V. Sick, M.N. Bui-Pham, R.L. Farrow, Opt. Lett. 20 (1995) 2036. [12] R.L. Farrow, M.N. Bui-Pham, V. Sick, in: Twenty-Sixth Symposium (International) on Combustion, The Combustion Institute, 1996, p. 975. [13] T.B. Settersten, R.L. Farrow, J.A. Gray, Chem. Phys. Lett. 369 (2003) 584. [14] T.J. Butenhoff, E.A. Rohlfing, J. Chem. Phys. 98 (1993) 5460. [15] R.N. Zare, Angular Momentum, Wiley, New York, 1988, p. 286. [16] S. Williams, E.A. Rohlfing, L.A. Rahn, R.N. Zare, J. Chem. Phys. 106 (1997) 3090. [17] G. Herzberg, Molecular Spectra and Molecular Structure III – Electronic Spectra and Electronic Structure of Polyatomic Molecules, Van Nostrand Reinhold, New York, 1966. [18] S.G. Westre, P.B. Kelly, Y.P. Zhang, L.D. Ziegler, J. Chem. Phys. 94 (1991) 270. [19] P. Botschwina, E. Schick, M. Horn, J. Chem. Phys. 98 (1993) 9215. [20] A.M. Mebel, S.-H. Lin, Chem. Phys. 215 (1997) 329. [21] J.F. Black, I. Powis, J. Chem. Phys. 89 (1988) 3986. [22] T. Etzkorn, J. Fitzer, S. Muris, J. Wolfrum, Chem. Phys. Lett. 208 (1993) 307. [23] I.R. Slagle, D. Gutman, J.W. Davies, M.J. Pilling, J. Phys. Chem. 92 (1988) 2455. [24] T.A. Reichardt, F. Di Teodoro, R.L. Farrow, S. Roy, R.P. Lucht, J. Chem. Phys. 113 (2000) 2263. [25] S. Roy, R.P. Lucht, T.A. Reichardt, J. Chem. Phys. 116 (2002) 571.
Coherent infrared–ultraviolet double-resonance spectroscopy of CH3 Thomas B. Settersten a
a,*
, Roger L. Farrow a, Jeffrey A. Gray
b
Combustion Research Facility, Sandia National Laboratories, P.O. Box 969, MS 9056, Livermore, CA 94551, USA b Department of Chemistry, Ohio Northern University, Ada, OH 45810, USA Received 25 November 2002; in final form 7 January 2003
Abstract Two-color polarization spectroscopy (TC-PS) and two-color resonant four-wave mixing spectroscopy (TC-RFWM) are used to detect photolytically produced CH3 radicals. An infrared laser pumps individual lines in the m3 fundamental ~ 2 A00 state, and an ultraviolet laser probes the pumped levels to reveal rotationally resolved spectra of transitions of the X 2 ~ 2 A0 state. The spectra are fit with a complex Lorentzian lineshape and yield an updated value of to the predissociated B 1 1 ~ state. A detection limit of 2 1013 CH3 molecules per cm3 per quantum state is 46 239:4 1:2 cm for T0 of the B observed for these coherent double-resonance techniques. Ó 2003 Elsevier Science B.V. All rights reserved.
1. Introduction Free methyl radical (CH3 ) is a primary intermediate in flames, plasmas, and photolytic reactions and is also observed in planetary atmospheres and interstellar clouds. Because of its chemical importance and structural simplicity, CH3 has been the subject of many spectroscopic studies. The development of optical techniques for the quantitative detection of CH3 remains an important research focus. Ultraviolet (UV) laser-induced fluorescence detection of CH3 is not possible because predissociation of its electronically excited states is rapid, and experimentalists have resorted to
*
Corresponding author. Fax: +1-925-294-2595. E-mail address: [email protected] (T.B. Settersten).
absorption-based techniques for optical detection of CH3 . Direct absorption in either the m3 vibrational fundamental in the mid-infrared [1–3] (IR) ~ 2 A0 ~ 2 A00 electronic system in the ulor the B X 1 2 traviolet [4–7] (UV) have been used for quantitative measurement of path-integrated CH3 concentration. Non-linear optical techniques such as REMPI [8,9], CARS [10], and DFWM [11,12] have been used for spatially resolved CH3 measurements. Spectral interferences in both the IR and UV regions, however, can cause large systematic uncertainties for all of these techniques. Although the ~ ~ origin band near 216 nm has predissociated B X a large peak cross-section, it lacks rotational structure, which complicates accurate simulation of the band contour under different conditions. In recent experiments, we demonstrated the viability of IR–UV double-resonance spectroscopy
0009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0009-2614(03)00062-9
T.B. Settersten et al. / Chemical Physics Letters 370 (2003) 204–210
for spatially resolved detection of the hydroxyl radical in complex reacting flows [13]. In the present work, we use the coherent versions of this technique, two-color polarization spectroscopy (TC-PS) and two-color resonant four-wave mixing (TC-RFWM) spectroscopy, for the detection of CH3 in moderate-to-high concentrations. The imposition of a double-resonance condition offers enhanced species selectivity and immunity to systematic biasing due to spectral interferences. We observe the first fully rotationally resolved spectra ~ –X ~ system of CH3 , which provide a precise of the B value for the vibronic origin and give direct evidence that the predissociation rate does not depend significantly on orientation of the rotating molecules.
2. Experiments The relevant energy level diagram for the double-resonance experiments is shown in Fig. 1 with the pump, probe, and signal transitions indicated by arrows. We label rotational quantum numbers ~ 2 A00 groundin the m3 ¼ 0 and m3 ¼ 1 levels of the X 2 00 00 0 0 state of CH3 as (N ; K ) and (N ; K ), respectively. IR transitions between these levels obey perpendicular selection rules DN ¼ 0 (K 6¼ 0), 1 and DK ¼ 1. Using the molecular constants determined by Amano et al. [1], our calculated IR transition frequencies agreed with all of our measured transition frequencies of the fully resolved
Fig. 1. Energy level diagram for IR–UV double resonance spectroscopy of CH3 .
205
DNK00 ðN00 Þ lines to within the pump laser bandwidth. The probe laser was tuned to transitions in ~ ~ origin band. We label rotational levels the B X 2 0 ~ in the B A1 Rydberg (3s) state of CH3 as (N ; K ) and denote probe transitions as DNK00 ðN00 Þ. The UV transitions obey parallel selection rules DN ¼ 0 (K 6¼ 0), 1 and DK ¼ 0. All of the double-resonance experiments described here involved transitions having common (N00 ; K00 ) levels as shown in Fig. 1. Methyl radicals were produced by flash photolysis of acetone or CH3 I using up to 30 mJ of the fourth-harmonic output of a 6-ns Q-switched Nd:YAG laser. All experiments were conducted at room temperature. For experiments at atmospheric pressure, a flow of argon (200 sccm) bubbled through acetone at 7°C (v.p. ¼ 100 Torr) and passed into the laboratory through a 50-mm diameter honeycomb that was used to stabilize the flow. The photolysis laser was formed into a horizontal sheet (1 40 mm) and directed through the gas flow. Production of CH3 was accompanied by loud acoustic bursts. For experiments at pressures below 1 Torr, CH3 I vapor was drawn through a 7-cm steel vacuum cube. Fused silica windows were mounted directly on two sides of the vacuum cube to allow the photolysis laser sheet (1 10 mm) to pass through the cell. Two 30-cm extensions were attached to other sides of the cube and capped with fused silica windows to allow the pump, probe, and signal beams to enter and exit the cell in a direction nominally perpendicular to the photolysis laser. The pump and probe lasers used in these experiments are similar to those described previously for double-resonance detection of OH [13]. The second harmonic output of a 1.5-ns pulsed Nd:YAG laser pumped two dye lasers. An IR pump beam tunable near 3.1 lm was generated by difference-frequency mixing the output of one of the dye lasers with the 532-nm output of the Nd:YAG laser. A UV probe beam tunable near 216 nm was generated by frequency tripling the output of the other dye laser. The pump and probe pulse energies each were monitored during the experiments using pyroelectric joule meters. The pump laser produced pulse energies of up to 500 lJ, while the probe laser produced pulse DK
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energies of up to 100 lJ. The wavenumber of the fundamental output from each dye laser was monitored and calibrated to within 0.1 cm1 . The pump and probe beams were crossed in the sample with respective beam diameters of approximately 1.5 and 1 mm. The optical paths were arranged so that the probe pulses reached the sample 0.25 ns after the pump pulses. For most experiments, the pump and probe laser pulses were delayed by 200 ns with respect to the photolysis pulse to allow for some thermalization. TC-PS experiments were performed with the pump beam polarized vertically and the probe beam polarized linearly at 45°. The beams crossed through the photolysis region at a full angle c ¼ 2° (Fig. 2a) and overlapped for approximately 4 cm. An a-BBO prism polarizer was placed in the UV beam after the sample and rotated to null the beam with an extinction ratio of 5 108 at 216 nm. The transmitted UV beam was focused through a 300lm diameter pinhole to reduce scattered light and then detected through UV-bandpass and neutraldensity filters by a UV-sensitive photomultiplier tube (PMT). The PMT output was integrated with a charge-integrating amplifier, digitized, and normalized by the probe pulse energy. Typically 100–500 laser shots were averaged for each measurement. TC-PS was not used to detect the CH3 produced by the photolysis of CH3 I because the vacuum cell windows severely degraded the polarization. TC-RFWM experiments were performed with all beams polarized vertically and propagating
Fig. 2. Experimental arrangement for (a) TC-PS and (b) TCRFWM.
forward in the same plane (Fig. 2b). The pump beam was split into two approximately equal-intensity beams that crossed at a full angle b ¼ 6:3°, resulting in an overlap of approximately 3 cm in the sample. Phase matching was achieved with the probe and signal beams emerging separated by 0.44°. For this geometry, the phase-matching bandwidth was more than sufficient to include all linked sets of spectral transitions [14]. Signals were detected as with TC-PS except that a 25-lm diameter pinhole was used to suppress scattered light. Additionally, several adjustable apertures were placed in the probe and signal beams to minimize this background.
3. Results and discussion 3.1. UV double-resonance spectra The first set of double-resonance experiments investigated TC-RFWM probe spectra with the pump laser tuned to particular IR transitions. Pump transitions in the r R3 or r R6 branches were strongest because the H€ onl–London factors are largest for the r R branches [15], and levels with K00 ¼ 3q (q is an integer) have double the statistical weight of the other K00 levels due to nuclear spin statistics. While double-resonance probe spectra at 1 atm appear nearly as broad as the entire absorption band, at reduced pressures collisional redistribution between (N00 ; K00 ) levels during the pump/probe interaction is negligible, and TCRFWM probe spectra observed at 1 Torr (Fig. 3) are well resolved into one set of P, Q, and R lines. The probe spectra are described by a line shape function LðmÞ that is equal to the modulus squared of a complex Lorentzian function: X A Dm 2 X A C 2 i i i i LðmÞ ¼ ð1Þ þ ; i Dm2i þ C2i i Dm2i þ C2i where Ai is the amplitude, Ci is the linewidth (HWHM), and Dmi is the detuning from each probe resonance i. The line shape function was derived using the theoretical treatment of Williams et al. [16]. For the TC-RFWM scheme employed here (Scheme 3 with YYYY polarization configu-
T.B. Settersten et al. / Chemical Physics Letters 370 (2003) 204–210
~ ~ origin Fig. 3. TC-RFWM probe spectra of the CH3 B X band at 1 Torr with the pump laser fixed on the r R3 (4) (bottom), r R3 (5) (middle), and r R3 (6) (top) transitions. Experimental data are shown as points. The solid curves represent the best-fits to the data using the line shape function given by Eq. (1).
ration in [16]), the signal results from ground-state gratings as well as from coherences between the excited states of the pump and probe transitions. In the present experiments, however, coherence ~ dephasing is dominated by predissociation in the B state [12], and the coherences between the excited states contribute negligibly to the RFWM signal because they are rapidly damped. The result is a single resonant denominator that corresponds to the ground-state grating. Eq. (1) accounts for interference occurring between multiple probe transitions. Inclusion of this complicating effect, which is also observed in other coherent techniques such as CARS, is essential to obtain meaningful fit parameters. We recorded and fitted with Eq. (1) probe spectra involving more than 20 different double resonances with the pump laser fixed on various isolated lines. The relative magnitudes of the P, Q, and R lines in each TC-RFWM spectrum (Ai in Eq. (1)) were constrained with calculated line
207
strengths [16]. The best-fit values for the frequencies and linewidths of the probe transitions are listed in Table 1. The precision of our measured ~ –X ~ rotational transitions is line positions for B limited by their large linewidths, and thus we cannot improve upon the established bond length ~ state that was inferred from structure of for the B ~ –X ~ origin band of CD3 [4,17]. By completely the B resolving these transitions for the first time, however, we can precisely determine the vibronic origin as T0 ¼ 46 239:4 1:2 cm1 from a weighted least-squares fit of computed line positions to measured line positions using fixed rotational ~ state (B ¼ 2C ¼ 8:827 constants [17] for the B cm1 ). Deviations of the fitted line positions from the computed positions are listed in Table 1. We estimate that a non-resonant background in our TC-RFWM spectra could cause an additional systematic uncertainty of up to 5 cm1 in our reported T0 value. Our result is compared with previous reports of T0 in Table 2. HerzbergÕs T0 value [17] represents the center of unresolved Q and P branches observed in absorption following flash photolysis of HgðCH3 Þ2 . More recently Westre et al. [18] determined T0 by fitting rotational resonance Raman excitation profiles. Te and T0 values have also been calculated by Botschwina et al. [19] and Mebel and Lin [20]. Westre et al. [18] measured linewidths for a series of N levels and used them to model the ~ potential barrier for predissociation of the B state. Their model suggests that widths for K ¼ N levels should be 10–20% smaller than widths for K ¼ 0 levels of the same N , although K was not resolved in their measurements. Our K -resolved widths generally reproduce the N
dependence observed by Westre et al. [18] and our precision is sufficient to confirm that the rotational predissociation rate depends weakly on molecular orientation (N ), decreasing <15% with increasing K . At low pressures TC-RFWM signals involving N00 ¼ K00 levels exhibited a short-lived, 20-fold enhancement when the polarization of the photolysis laser was parallel to the (vertical) pump and probe lasers. This observation indicated a preferential orientation of the CH3 photofragments with rotation around the C3 top axis, and is consistent
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Table 1 Observed coherent IR–UV double-resonance transitions for CH3 Pump transition
Wavenumber (cm1 )
Probe transition
Observed wavenumber ð2rÞ (cm1 )
O–Ca (cm1 )
r
3211.5 3211.5 3211.5 3211.5 3209.6 3209.6 3209.6 3199.7 3199.7 3217.7 3217.7 3217.7 3217.7 3217.7 3217.7 3217.7 3217.7 3217.7 3235.5 3235.5 3235.5 3235.5 3235.5 3235.5 3216.2 3282.5 3282.5 3252.9 3252.9 3224.3 3224.3 3224.3 3270.0 3270.0 3232.3 3248.1 3248.1
P0 (2) R0 (2) P0 (2) R0 (2) P2 (3) Q2 (3) R2 (3) Q3 (3) Q3 (3) P3 (4) Q3 (4) R3 (4) P3 (4) Q3 (4) R3 (4) P3 (4) Q3 (4) R3 (4) P3 (5) Q3 (5) R3 (5) P3 (5) Q3 (5) R3 (5) Q5 (5) P0 (6) R0 (6) P3 (6) R3 (6) Q6 (6) Q6 (6) Q6 (6) P3 (7) R3 (7) Q7 (7) Q9 (9) Q9 (9)
46 194.1 (6.3) 46 288.7 (7.4) 46 201.7 (4.4) 46 293.5 (3.3) 46 235.1 (6.1) 46 156.8 (6.4) 46 216.5 (4.1) 46 235.6 (1.4) 46 244.0 (4.2) 46 158.4 (7.6) 46 221.2 (7.4) 46 319.6 (7.3) 46 163.5 (4.6) 46 227.6 (2.7) 46 323.8 (5.2) 46 152.2 (5.2) 46 223.4 (2.5) 46 319.9 (2.9) 46 134.5 (6.7) 46 225.4 (4.9) 46 321.9 (2.5) 46 139.0 (6.7) 46 223.3 (4.9) 46 330.1 (2.5) 46 228.7 (2.9) 46 109.0 (11.5) 46 338.9 (4.8) 46 104.0 (2.6) 46 328.7 (2.8) 46 222.9 (2.7) 46 217.9 (3.7) 46 225.8 (2.7) 46 071.9 (4.6) 46 343.1 (7.9) 46 220.4 (5.0) 46 200. (3.3) 46 197.2 (3.6)
)5.5 0.8 2.1 5.6 9.0 7.7 5.9 1.4 9.8 0.7 )7.1 3.0 5.8 )0.7 7.2 )5.5 )4.9 3.3 1.7 4.3 )5.1 6.2 2.2 3.1 1.2 5.7 6.1 )2.6 )7.4 )0.3 )5.3 2.6 )7.4 )1.0 2.2 )6.4 )9.3
R0 (2) R0 (2) r R0 (2) r R0 (2) r R2 (3) r R2 (3) r R2 (3) r R3 (3) r R3 (3) r R3 (4) r R3 (4) r R3 (4) r R3 (4) r R3 (4) r R3 (4) r R3 (4) r R3 (4) r R3 (4) r R3 (5) r R3 (5) r R3 (5) r R3 (5) r R3 (5) r R3 (5) r R5 (5) r R0 (6) r R0 (6) r R3 (6) r R3 (6) r R6 (6) r R6 (6) r R6 (6) r R3 (7) r R3 (7) r R7 (7) r R9 (9) r R9 (9) r
a
84 (17) 103 (19) 79 (10) 82 (9) 83 (13) 76 (14) 93 (9) 86 (4) 101 (13) 80 (18) 94 (20) 103 (16) 80 (10) 74 (7) 110 (10) 81 (9) 70 (6) 84 (6) 106 (12) 69 (10) 80 (7) 98 (9) 75 (9) 88 (6) 98 (10) 112 (25) 86 (7) 98 (7) 97 (11) 93 (14) 88 (14) 81 (15) 111 (18) 93 (20) 78 (12) 104 (15)
Probe wavenumber calculated from best-fit T0 .
Table 2 B–X vibronic origin values for CH3 T0 ðcm1 Þ
Reference
46 239:4 1:2 46 300 50 46 205 46 027a 46 435
This work [18] [17] [19] [20]
a
FWHM ð2rÞ (cm1 )
Adjusted from Te using calculated zero-point energies [20].
with the dissociation process proceeding in C3v symmetry. In this case, top-axis rotation of the methyl iodide parent molecule results in preferential N00 ¼ K00 orientation of the photofragment [21]. At 0.7 Torr the enhancement was observed to decay in approximately 10 ns. The fast collisional reorientation rate is consistent with large nonthermal velocities of CH3 fragments following photodissociation of CH3 I with 266-nm photons [9].
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3.2. IR double-resonance spectra Pump spectra were recorded with the probe laser fixed at 46 200 cm1 , where most of the Q-branch lines overlap. A TC-PS signal obtained from CH3 at 1 atm is plotted as a function of pump frequency in Fig. 4. This polarization spectrum consists of three isolated CH3 transitions in the same region where lines from stable molecules such as CH4 or H2 O can overlap badly in direct IR-absorption spectra [3]. The relatively large intensity of the r R6 (6) line is expected because of the larger population of the N00 ¼ K00 ¼ 6 level at 293 K and the quadratic dependence of TC-PS signals on molecule density [18]. The observed TC-PS linewidths were consistent with the pump laser bandwidth ( 0.2 cm1 ) and a Lorentzian-squared line shape function, and destructive interference was evident between the closely spaced r R6 (6) and p Q7 (7) lines. We did not analyze fits to the pump spectra, however, because the resonances in the m3 band are already accurately known, and rapid collisional redistribution among quantum states during the pump/probe interaction makes it difficult to absolutely quantify the rotational line intensities. 3.3. Saturation and detection limits Optical saturation of the double-resonance signals was evident at modest laser intensities. The
Fig. 4. Rotational transitions in the m3 fundamental band of CH3 observed using flash photolysis/TC-PS at atmospheric pressure. The solid curve represents a best-fit profile using a Lorentzian-squared line shape function.
209
pump laser saturation irradiance was approximately 5 MW/cm2 for TC-PS at 1 atm, which is similar to the value observed for OH in a 1-atm flame [18], and approximately 0.7 MW/cm2 for TC-RFWM at 1 Torr. The probe laser saturation irradiance was approximately 11 MW/cm2 for TCPS at 1 atm and 0.9 MW/cm2 for TC-RFWM at 1 ~ state is on the Torr. Because the lifetime of the B order of 104 times shorter than the laser pulse, stimulated emission was insignificant, and probe saturation is best understood as depletion of the ground-state population. We observed signal-to-noise ratios that were on the order of 300 for TC-PS at 1 atm and 30 for TCRFWM at 1 Torr, both of which were somewhat smaller than observed for OH in a 1-atm flame [18]. Detection limits were based on measurements of CH3 concentration resulting from direct absorption of the probe beam [11,12,22]. A background-corrected absorption spectrum and the simulated spectrum are shown in Fig. 5. The poor agreement at the R-branch head may indicate that the CH3 did not reach thermal equilibrium with the bath gas. The observed peak absorption corresponds to a molecule density that is on the order of 1016 cm3 , which is consistent with the 1-ls half life for CH3 we observed and the known rate constant for self reaction at 300 K [23]. We estimated that the molecule density resulting from low-pressure photolysis of CH3 I was approximately 30% of that observed when acetone was photolyzed at atmospheric pressure. Based on
Fig. 5. The observed absorption spectrum of CH3 B–X origin band at 1 atm following photolysis of acetone. The solid curve is the band contour simulated at 293 K using our T0 value and a linewidth of 80 cm1 .
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these estimates, TC-PS and TC-RWFM have a detectivity limit of 2 1013 CH3 molecules per cm3 per quantum state. 4. Conclusions We have demonstrated the use of coherent IRUV double-resonance techniques for the detection of CH3 at room temperature. The state specificity afforded by the double-resonance condition resulted in the first fully rotationally resolved UV ~ –X ~ origin band of CH3 , enabling spectra in the B direct spectroscopic characterization of the pre~ state. While these new coherent dissociated B techniques are not nearly as sensitive as CRDS [3,7], they have advantages as diagnostics for complex, reacting flows at higher pressures because of their higher spatial resolution and analyte specificity. We expect that TC-PS and TC-RFWM spectroscopy of CH3 and other non-fluorescing free radicals will be significantly improved using single-mode picosecond lasers. The expected reductions of collisional depolarization [24,25], and shot-to-shot fluctuations of the laser will dramatically improve signal-to-noise ratios. Acknowledgements The authors appreciate the excellent laboratory assistance of Mr. Paul Schrader and support from the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences. References [1] T. Amano, P.F. Bernath, C. Yamada, Y. Endo, E. Hirota, J. Chem. Phys. 77 (1982) 5284. [2] G.A. Bethardy, R.G. Macdonald, J. Chem. Phys. 103 (1995) 2863.
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