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High-resolution diode laser absorption spectroscopy of the O–H stretch overtone band (2,0,0)←(0,0,0) of the HO2 radical
Journal of Molecular Spectroscopy 219 (2003) 163–169 www.elsevier.com/locate/jms
High-resolution diode laser absorption spectroscopy of the O–H stretch overtone band ð2; 0; 0Þ ð0; 0; 0Þ of the HO2 radicalq John D. DeSain,a,* Andrew D. Ho,b and Craig A. Taatjesb b
a The Aerospace Corporation, M5/754, P.O. Box 92957, Los Angels, CA 90009-2957, USA Combustion Research Facility, Sandia National Laboratories, Mail Stop 9055, Livermore, CA 94551-0969, USA
Received 5 November 2002; in revised form 23 December 2002
Abstract The O–H stretching overtone (2m1 ) of the HO2 radical was observed between 6603.2 to 6685:5 cm1 by using tunable diode laser absorption spectroscopy (TDLAS). About 1000 lines were observed in this region of which 491 transitions could be definitively assigned to the 2m1 . The spectrum is observed to be an A/B hybrid band with band features of both a perpendicular and parallel nature. Transitions of the A-type bands with Ka0 ¼ 0–3, N 0 6 16 and transitions of the B-type bands with Ka0 ¼ 0; 1, N 0 6 15 were assigned. The origin calculated from the best fit to the present spectrum is at 6651:1876ð38Þ cm1 which is 4:6 cm1 higher than previously reported. The overtone spectrum is observed to be heavily perturbed, possibly by Fermi resonance with energy levels of the nearby ðm2 þ 5m3 Þ state. Ó 2003 Elsevier Science (USA). All rights reserved. Keywords: HO2 radical; Overtone; Infrared spectroscopy; Diode laser
1. Introduction Optical monitoring of the hydroperoxyl radical (HO2 ) is of great interest because of the importance of HO2 as a reaction intermediate in both atmospheric and combustion chemistry. The HO2 radical has been monitored previously by laser absorption spectroscopy (LAS) utilizing absorption transitions in the m3 region [1–3]. Currently, laser monitors of the m3 region use liquid nitrogen cooled Pb-salt diodes as the infrared source. However, non-cryogenic diode lasers are commercially available in the spectral region of the O–H stretching vibrational overtone of the m1 fundamental (near 1.51 lm). The frequency range of the overtone transition is also convenient for environmental monitoring as it falls in one of the ‘‘atmospheric windows’’ and thus could possibly provide a means for measurement of HO2 in the atmosphere. Transitions in this overtone region have recently been used to monitor HO2 q
Supplementary data for this article are available on ScienceDi-
rect. * Corresponding author. Fax: 1-310-336-7680. E-mail address: [email protected] (J.D. DeSain).
in gas phase chemical reactions [4,5]. However, although room temperature absorption linestrengths have been reported [6,7], the HO2 overtone spectrum has not previously been observed and assigned at sufficiently high resolution to identify the nature of the probe transition in those experiments. The HO2 radical is a prototypical light asymmetric top radical. Because of its importance it has been studied by several different spectroscopic methods. Chance et al. [8] obtained an accurate set of ground state constants by combining their far infrared measurements with previous microwave [9] and millimeter [10] measurements. The three ground state fundamentals m1 , m2 , and m3 of HO2 have been observed and assigned by using difference frequency absorption [11], diode laser absorption [12], and Fourier transform spectroscopy [13]. Also recently Fink and Ramsay [14] have observed and assigned the near-infrared emission spectrum of the A~2 A0 ! X~ 2 A00 electronic transition at high resolution by Fourier transform spectrometry. Fink and Ramsay performed a new fit of the ground state which included both their work and the previous work of Chance et al. [8] The Fink and Ramsay experiment includes observation of emission from HO2 in the spectral region of
0022-2852/03/$ - see front matter Ó 2003 Elsevier Science (USA). All rights reserved. doi:10.1016/S0022-2852(03)00022-5
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10 000–5500 cm1 . This region includes the region around 6600 cm1 where emission from the first overtone of m1 lies. However, Fink and Ramsay have yet to publish an assignment of this overtone spectrum or the line positions of transitions in this region. Emission from the first overtone was observed previously at medium resolution (0:2 cm1 ) by Tuckett et al. [15] with a SISAM spectrometer. Both the perpendicular B-type transitions and parallel A-type transitions were observed by Tuckett et al. with the predominant band being the A-type. The A/B hybrid nature of the transition is a result of the O–H stretching motion being in the a; b plane of the molecule. The stretching thus generates an alternating dipole along the a and b axes. Hybrid bands have not previously been observed in either m1 , m2 , or m3 . Only B-type transitions were reported by Yamada et al. [11] for the m1 fundamental, whereas only A-type transitions were observed by Burkholder [13] for m2 and m3 . Tuckett et al. [15] observed the Q branches of the DK ¼ 0 transitions as bandheads with the intensity rapidly falling off with N 0 . Asymmetry splitting was observed in only the Ka0 ¼ 1 transitions and the spin-splitting of the subbands was not resolved. Tuckett et al. fit their assignments of their 0:2 cm1 -resolution spectrum to a 7 term Hamiltonian and reported values for the upper state rotational constants A, B, C, DK , and DN , locating the origin at 6646:46 0:15 cm1 . The lack of observed perturbations by Tuckett et al. in the overtone spectrum is interesting given that Barclay et al. [16,17] predict the 2m1 to be in Fermi resonance with the (m2 þ 5m3 ). Using the three-dimensional potential energy surface of Walch and Duchovic. [18] Barclay et al. calculated the vibrational wavefunctions of HO2 in an attempt to identify low-lying Fermi-resonant pairs. The origin of the 2m1 and (m2 þ 5m3 ) were predicted by Barclay et al. to be only 7:3 cm1 apart. The Fermi coupling between these two states was predicted to be significant with the predicted coupling matrix element ¼ 3:2 0:2 cm1 . The coupling is strong enough that Barclay et al. suggest that ð0; 1; 5Þ ! ð0; 0; 0Þ transitions might be observed in the 2m1 spectrum. In this work the high-resolution infrared spectrum of HO2 is measured from 6603.2 to 6685:5 cm1 by using diode laser frequency modulation spectroscopy. The first overtone of the O–H stretch of HO2 is observed and assigned at a much higher resolution (0:0003 cm1 ) than previously reported. The spectrum is observed to be an A/B hybrid band with both A-type and B-type transitions. The spectrum is found to be heavily perturbed. These perturbations make accurate assignment of upper state rotational constants difficult. A ‘‘best’’ fit to the assigned spectrum is presented fitting 390 transitions of the Ka0 ¼ 0–3, with N 0 up to 16. The rotational constants obtained from this ‘‘best’’ fit to the high-resolution spectrum are in significant disagreement with those reported by Tuckett et al. [15] on the basis of
their lower-resolution spectrum. The origin in the present work is found about 4:6 cm1 higher in energy than reported by Tuckett et al. The high-resolution assignment presented here should allow the temperature dependence of absorption cross-sections to be more accurately predicted. This should make the monitoring of HO2 by using the 2m1 transition more applicable to sensing in a variety of environments.
2. Experiment The pulse photolysis/long path absorption method used in this experiment is similar to that employed in previous experiments [4,19]. The infrared spectrometer uses a narrow linewidth (<300 kHz) cw tunable commercial infrared diode laser. The diode laser can be scanned continuously by 0:0003 cm1 (on average) frequency steps at the highest resolution for slightly more than 1 cm1 (30 GHz). The tuning range is limited by the scanning PZT of the laser head. Therefore the recorded HO2 spectrum is taken as a series of 30 GHz scans. Two beam splitters are used to separate the laser output into three separate beam paths. The first beam splitter is used to place part of the output through a 150 MHz stable etalon. The diode laser has a considerable non-linearity in the frequency step-size; the step-size can nearly double in magnitude across the 30 GHz tuning range. The fixed etalon is used as a frequency marker and allows the removal of the non-linearity found in the diode laser frequency step-size while scanning the laser at high resolution. The etalon is scanned with the laser frequency fixed to insure single mode laser operation before and after each wavelength scan. The second beam splitter is used to direct part of the laser output through a reference gas cell. The cell has a path length of 85 cm and is filled with 8–9 Torr of NH3 . The high-resolution spectrum of NH3 from 6400 to 6600 cm1 has previously been observed by Fourier transform spectrometry to an accuracy of 0:0005 cm1 [20]. The spectrum of NH3 is obtained simultaneously with the HO2 spectrum and is used to calibrate the absolute frequency of the HO2 spectrum. After the calibration procedure the transitions in the reference NH3 spectrum have a standard deviation of 0:0027 cm1 to those previously observed by Fourier transform spectrometry. The infrared (IR) laser output is passed 20 times through a Herriott-type flow cell [19]. The IR probe beam enters into the 1.4 m long flow cell through a CaF2 window and then enters the multipass cavity off axis through a notch in the back Herriott mirror and is multipassed through the flow cell. After exiting through a notch in the front Herriott mirror and passing through another CaF2 window, the probe beam is passed through a band pass filter and then focused onto a
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detector. The UV photolysis beam passes through the center of the front Herriott mirror and travels on axis through the quartz flow cell before passing through the center of the back Herriott mirror and exiting the cell. The total effective length of the region where the IR probe beam and UV photolysis beam overlap is 20 m. To increase the signal to noise ratio of the observed spectrum, two-tone frequency modulation of the infrared laser is employed. The HO2 is formed from the reaction of O2 with CH2 OH. Cl is generated by photolysis at 355 nm of Cl2 and CH2 OH is subsequently generated by the reaction of Cl with methanol. The CH2 OH radical then reacts with O2 to produce the HO2 radical and formaldehyde. This reaction produces a 100% yield of HO2 as a product [21]. The formation of HO2 by this method avoids the formation of OH that complicated the spectrum recorded by Tuckett et al. [15]. The overtones of formaldehyde [22] and methanol [23] are not expected to contribute in this region. Typical gas concentrations are ½O2 ¼ 1:2 1017 cm3 , ½Cl2 ¼ 3:1 1015 cm3 , and ½CH3 OH ¼ 5:5 1015 cm3 . Helium is added to a total density of 5:18 1017 cm3 .
3. Analysis The observed spectrum has 1000 lines between 6603.2 and 6685:5 cm1 . A significant number of lines was expected given that transitions may be split by both asymmetry doubling and spin-rotation interaction. This splitting can lead to a maximum of 12 branches for Atype transitions and 24 branches for B-type transitions. The ground state rotational constants obtained by Fink and Ramsay can be used to calculate the ground state combination differences of HO2 . A computer program was then used to search through the spectrum for pairs of observed lines whose difference in energy (frequency) matched one of the ground state combination differences for the P and R of a common Ka0 value for the A-type transitions. The threshold for accepting a line pair as matching a ground state combination difference was set to (observed frequency difference – ground state combination difference) < 0:0120 cm1 . Pairs of lines that matched a ground state combination difference were then assigned the appropriate quantum numbers. These transitions could then be calculated by using the upper state constants of Tuckett et al. [15] and a modified version of the asymmetric top spectral fitting program of Sears [24] that utilizes the Watson A-reduced Hamiltonian with spin-rotation interaction. The residuals (observed – predicted) were then plotted against the N 0 quantum number to identify Ka0 series. The search was done starting with Ka0 ¼ 0 R and P transitions, but no clear series could be identified in the residuals plot. However, this particular series could be identified
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without the search program, as these transitions are some of the most intense in the observed spectrum. The upper state constants of Tuckett et al. [15] were then modified by doing a preliminary fit of the upper state that used the observed Ka0 ¼ 0 R, Q, and P branches. The most notable change is a 4:6 cm1 shift in the origin and a significant increase in the DN constant. These new preliminary upper state constants were used in the searching program to identify the R and P branches of a common Ka0 . Fig. 1 shows an example of a scatter plot that results from the searching program. The residuals calculated for lines matching the R and P Ka0 ¼ 1 NK0 0 ;N 0 ðK 0 þ1Þ F1 ground state combination difa a ferences are seen in the plot. While there are lots of line pairs that may coincidentally match the Ka0 ¼ 1 R and the P ground state combination difference in this spectral region the branches are clearly identifiable up to N 0 ¼ 14 in the scatter plot. Once the A-type transitions were identified by this program, B-type transitions could also be easily located by using ground state combination differences. Using this approach the A-type bands with Ka0 ¼ 0–3, N 0 6 16 and the B-type bands with Ka0 ¼ 0; 1 N 0 6 15 could be identified in the HO2 spectrum. In most cases an assigned transition for the Ka0 ¼ 0 or 1 series will fit at least 3 different ground state combination differences and in many cases more. A total of 491 transitions have been identified in the observed spectrum. These transitions have 554 unique ground state combination differences with a standard deviation of 0:0059 cm1 from those predicted by using the ground state constants of Fink and Ramsay.
Fig. 1. Residuals for observed transitions to a prediction of the transition frequency calculated by using the Watson A-reduced Hamiltonian versus the N 0 value. These residuals are for pairs of observed lines that have the correct ground state energy difference for transitions involving the R and P branch of Ka0 ¼ 1 NK0 0 N 0 ðK 0 þ1Þ F1 . The line pairs a a were found by a spectral searching program and fit by using preliminary upper state constants as described in the text. The ‘‘correct’’ Ka0 ¼ 1 series can easily be identified from the plot.
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Fig. 2. High-resolution spectrum of HO2 m1 first overtone showing the portions of the A-type band that was previously used to monitor HO2 for chemical kinetics [4,5,7]. The asymmetry splitting of the q P1 (11) is resolvable with the + and ) parity components labeled. The spin-rotation spitting is not resolvable for q P1 (11) or q P2 (9).
Fig. 2 shows a portion of the A-type transitions in the spectrum. The transition at 6625:7773 cm1 has previously been used to monitor HO2 formation from gas phase chemical reactions [4,5,25]. The observed spectrum has many features similar to those observed previously in the spectrum of Tuckett et al. [15]. The intensity of the A-type Q branches decrease rapidly with increasing N 0 . Except for Ka0 ¼ 0, the asymmetry splitting is observed to increase with increasing N 0 but decrease rapidly with increasing Ka0 . Tuckett et al. [15] were unable to resolve the spin splitting, but this spin splitting is resolvable at the higher resolution of the present study. The spin splitting of Ka0 ¼ 0 are not resolved, while the Ka0 ¼ 1, 2, and 3 exhibit spin splitting. The spin splitting in these series tends to decrease as N 0 increases. Fig. 3 shows an example of the B-type Ka0 series in the
spectrum. Both spin states of the R Q0 (N) are clearly observable in the spectrum. The Q branches with different Ka0 are widely spaced for the B-type transitions, so that only those transitions corresponding to Ka0 ¼ 0 and 1 have been identified. The searching program calculated the transitions in the spectrum using a Watson A-reduced Hamiltonian for an asymmetric-top that included spin-rotation and centrifugal distortion terms previously used by Fink and Ramsay [14]. Additional Hamiltonian terms are needed to adequately describe the energy levels of the overtone that are affected by Fermi resonance with the (m2 þ 5m3 ) levels. The lack of foreknowledge of the (m2 þ 5m3 ) upper state makes such a fit difficult. There are a large number of low intensity transitions observed in this spectral region, but there has yet to be an assignment of any of these transitions to the ðm2 þ 5m3 ). Generally, if the mixing of two upper state energy levels is locally confined to only a small number of N 0 states of a few Ka0 series then it is possible to produce an adequate fit to a Hamiltonian by just giving zero weight to those transitions that experience perturbation. Unfortunately, all the Ka0 series in the present spectrum appear to have some type of perturbation. Nonetheless, by leaving out both the Ka0 ¼ 1 NK0 a0 ;N 0 Ka0 F1 and F2 and the Ka0 ¼ 2 NK0 a0 ;N 0 Ka0 F1 and F2 , N 0 > 6 asymmetry branches, a final fit could be made that produces a semi-quantitative description of the observed spectrum. In total 390 transitions were used in the final fit with an average residual of 0:0046 cm1 . The resulting parameters from such a fit are of uncertain physical significance because several of the resulting parameters are of an unrealistically large magnitude. The resulting parameters are listed in Table 1. All 491 assigned transitions and their residuals to the fit are listed in the supplementary material.
4. Discussion
Fig. 3. High-resolution spectrum of HO2 m1 overtone showing both spin components of the R Q0 branch of the B-type band. Several transitions of the A-type band can also be seen in the spectrum.
The electronic band of HO2 A~2 A0 ! X~ 2 A00 ð0; 0; 0Þ ! ð0; 0; 0Þ was also observed and assigned by Tuckett et al. [15] using the same apparatus that observed the overtone band. This band was observed again by Fink and Ramsay [14] at much higher resolution and the assignments of Tuckett et al. were found to be essentially correct. The main deviation between the two studies was that the transitions observed by Tuckett et al. were found by Fink and Ramsay to be systematically lower by about 0:2 cm1 . The origin of the overtone band reported here is found to be different than Tuckett et al. by 4:6 cm1 . The origin found here is similar to the origin determined by Becker et al. at low resolution (6653 10 cm1 ) [26]. Since there are significant changes in many of the upper state centrifugal distortion terms, the deviation between the assignments presented here and assignments of Tuckett et al. are not simply a
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Table 1 Molecular constants of HO2 in MHz. Constant
2m1 (This work)
2m1 Tuckett et al. [15]
mo A B C DN DNK DK dN dK UN UNK UKN UK LK 108 eaa ebb ecc 1=2ðeabþ eba Þ DSK DSNK dSN USK
6651:1876ð38Þ cm1 566 706(50) 33 646(114) 31 501(62) 0.867(36) )12.18(84) 186.0(74) )0.196(11) 45(11) )0.00097(85) 0.0534(38) )0.465(70) 0.099 14(18)a )0.0001387(18)a )44 861(84) )356(84) )60(130)
6646:59ð15Þ cm1 545 982(330) 33 486(27) 31 676(27) 0.156(45) 3.60a 95.9(18)
a
83(36) )20(9) )9(9) )0.030 39(92)a
Ground state [27] 610 273.106(25) 33 517.733(15) 31 667.726(15) 0.116 865(10) 3.445 1(24) 123.572 4(35) 0.006 149 9(26) 1.977 9(75) 0.00001936(44) 0.00106(13) 0.099 14(18) )0.0001387(18) )49 572.006(113) )422.934(84) 8.748(84) )194.39(36) 23.664(21) 0.291(22) 0.159(12) )0.030 39(92)
Held fixed to the ground state value.
systematic offset. The previous assignment by Tuckett et al. [15] (Ka0 ¼ 0–4, N 00 up to 21) could be fit to a simple 7 term Hamiltonian. Given the perturbations expected in the spectrum the simplicity of the previous fit is somewhat surprising. As stated earlier the upper state constants of Tuckett et al. could not be used to identify the initial Ka0 ¼ 0 series observed at high resolution. The Ka0 series reported here are found to deviate somewhat even from the ‘‘best’’ fit to a much larger Hamiltonian than that used by Tuckett et al. As seen in Table 1 several resulting Hamiltonian parameters differ significantly from those determined by Tuckett et al. However, many of the qualitative features of the band described by Tuckett et al. do agree well with the observed highresolution spectrum. These features include the overtone band having significant intensity in both the A-type and the B-type transitions, the relative weakness of the Atype Q branches, and the Ka0 ¼ 1 branches being clearly asymmetry-split at low N 0 . It is possible that the more intense OH transitions in this region obscured too much of the spectrum observed by Tuckett et al. for an adequate assignment. Fink and Ramsay [14] have also speculated that the spectra measured by Tuckett et al. using a SISAM spectrometer was affected by instrumental artifacts. Given the lack of agreement between Tuckett et al. and this study for the band origin, rotational constants and amount of perturbations it can be concluded that the assignment of the overtone band reported here does not agree quantitatively with the previous assignment. A standard Watson A-reduced Hamiltonian for an asymmetric rotor is not adequate to reproduce the ob-
Fig. 4. Residuals for the Ka0 ¼ 1 NK0 a0 N 0 Ka0 F1 R branch as a result of the ‘‘best’’ fit to the HO2 m1 overtone spectrum. The series deviates significantly from the ‘‘best’’ fit to the spectrum and was given a weight of zero in the final fit.
served spectrum for the overtone. Fig. 4 shows the residuals for the R branch of the Ka0 ¼ 1 NK0 a0 ;N 0 Ka0 F1 that was left out of the ‘‘best’’ fit. It can be seen that even the ‘‘best’’ fit of the assigned spectrum does a poor job of reproducing most of the observed transitions in this particular Ka0 series. The average deviation of the observed spectrum to the fit is significantly better than this one series. As seen in Table 1 several of the higher order rotational constants needed to fit the upper state are significantly larger than those needed to fit the ground state. The centrifugal distortion constants ðDN , DNK , dN , UN , UNK , UKN ) involving N are noticeably larger than
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the ground state or those obtained by Hirota et al. for the unperturbed m1 . The cause of such large N dependent distortion in the upper state could be due to these series walking into resonance with nearby energy levels from the ðm2 þ 5m3 ) as N increases. Since the degree of overlap is unlikely to be the same for each Ka0 series it is perhaps fortuitous that such a good qualitative fit can be made at all. A more accurate representation of the overtone spectrum would require a Hamiltonian that includes terms that account for the Fermi resonance as well as knowledge of the upper state energy levels of the ðm2 þ 5m3 ). Thus while the parameters presented here may have limited physical significance, they do at least provide a concise summary of the experimental data. None of the ground state fundamentals have been observed to have the hybrid appearance of the O–H stretching overtone. The reported absence of the A-type bands in the m1 is particularly interesting given that as seen in Fig. 3 both the A-type and B-type transitions have significant intensity in the overtone spectrum. Most of the assigned transitions in the Yamada et al. [11] paper are of the B-type Q branches. It appears that only those B-type R and P transitions that run through the spectral regions very near these Q branches were assigned. The A-type Q branches in the overtone are weak. The strongest transitions of the A-type should therefore be in these R- and P-branch regions of the m1 spectrum that were not assigned by either Yamada et al. [11] or Burkholder et al. [13] Given that both m1 and 2m1 transitions involve motion of the hydrogen in the a/b plane and that both band types are so clearly observed in the overtone spectrum it would be interesting to reinvestigate the m1 for these A-type bands.
5. Conclusion The high-resolution infrared spectrum of HO2 has been observed from 6603.2 to 6685:5 cm1 by using diode laser frequency-modulation spectroscopy. The first overtone of the O–H stretch of HO2 has been observed and assigned for Ka0 ¼ 0–3, N 0 6 16. Both A-type and B-type transitions are observed in the spectrum. The spectrum is found to fit poorly to the Hamiltonian used in previous studies to fit the ground state and m1 fundamental. This is most likely due to the Fermi resonance predicted to exist between the overtone and the ðm2 þ 5m3 ). Given the degree of perturbation in the spectrum it seems likely that some weak ð0; 1; 5Þ ð0; 0; 0Þ transitions are indeed in the recorded spectrum. If these transitions can be identified, the degree of mixing between the two states may be able to be determined. The high-resolution assignment presented here will allow for the obtainment of accurate absolute number densities of the HO2 transitions at various temperatures. This should make the monitoring of HO2
by using the 2m1 more useful in remote sensing and kinetics applications.
Acknowledgments J.D.D. would to thank Prof. Robert F. Curl for useful discussions on this manuscript. This work is supported by the Division of Chemical Sciences, Geosciences, and Biosciences, the Office of Basic Energy Sciences, the US Department of Energy. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contact DE-AC0494-AL85000.
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High-resolution diode laser absorption spectroscopy of the O–H stretch overtone band ð2; 0; 0Þ ð0; 0; 0Þ of the HO2 radicalq John D. DeSain,a,* Andrew D. Ho,b and Craig A. Taatjesb b
a The Aerospace Corporation, M5/754, P.O. Box 92957, Los Angels, CA 90009-2957, USA Combustion Research Facility, Sandia National Laboratories, Mail Stop 9055, Livermore, CA 94551-0969, USA
Received 5 November 2002; in revised form 23 December 2002
Abstract The O–H stretching overtone (2m1 ) of the HO2 radical was observed between 6603.2 to 6685:5 cm1 by using tunable diode laser absorption spectroscopy (TDLAS). About 1000 lines were observed in this region of which 491 transitions could be definitively assigned to the 2m1 . The spectrum is observed to be an A/B hybrid band with band features of both a perpendicular and parallel nature. Transitions of the A-type bands with Ka0 ¼ 0–3, N 0 6 16 and transitions of the B-type bands with Ka0 ¼ 0; 1, N 0 6 15 were assigned. The origin calculated from the best fit to the present spectrum is at 6651:1876ð38Þ cm1 which is 4:6 cm1 higher than previously reported. The overtone spectrum is observed to be heavily perturbed, possibly by Fermi resonance with energy levels of the nearby ðm2 þ 5m3 Þ state. Ó 2003 Elsevier Science (USA). All rights reserved. Keywords: HO2 radical; Overtone; Infrared spectroscopy; Diode laser
1. Introduction Optical monitoring of the hydroperoxyl radical (HO2 ) is of great interest because of the importance of HO2 as a reaction intermediate in both atmospheric and combustion chemistry. The HO2 radical has been monitored previously by laser absorption spectroscopy (LAS) utilizing absorption transitions in the m3 region [1–3]. Currently, laser monitors of the m3 region use liquid nitrogen cooled Pb-salt diodes as the infrared source. However, non-cryogenic diode lasers are commercially available in the spectral region of the O–H stretching vibrational overtone of the m1 fundamental (near 1.51 lm). The frequency range of the overtone transition is also convenient for environmental monitoring as it falls in one of the ‘‘atmospheric windows’’ and thus could possibly provide a means for measurement of HO2 in the atmosphere. Transitions in this overtone region have recently been used to monitor HO2 q
Supplementary data for this article are available on ScienceDi-
rect. * Corresponding author. Fax: 1-310-336-7680. E-mail address: [email protected] (J.D. DeSain).
in gas phase chemical reactions [4,5]. However, although room temperature absorption linestrengths have been reported [6,7], the HO2 overtone spectrum has not previously been observed and assigned at sufficiently high resolution to identify the nature of the probe transition in those experiments. The HO2 radical is a prototypical light asymmetric top radical. Because of its importance it has been studied by several different spectroscopic methods. Chance et al. [8] obtained an accurate set of ground state constants by combining their far infrared measurements with previous microwave [9] and millimeter [10] measurements. The three ground state fundamentals m1 , m2 , and m3 of HO2 have been observed and assigned by using difference frequency absorption [11], diode laser absorption [12], and Fourier transform spectroscopy [13]. Also recently Fink and Ramsay [14] have observed and assigned the near-infrared emission spectrum of the A~2 A0 ! X~ 2 A00 electronic transition at high resolution by Fourier transform spectrometry. Fink and Ramsay performed a new fit of the ground state which included both their work and the previous work of Chance et al. [8] The Fink and Ramsay experiment includes observation of emission from HO2 in the spectral region of
0022-2852/03/$ - see front matter Ó 2003 Elsevier Science (USA). All rights reserved. doi:10.1016/S0022-2852(03)00022-5
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10 000–5500 cm1 . This region includes the region around 6600 cm1 where emission from the first overtone of m1 lies. However, Fink and Ramsay have yet to publish an assignment of this overtone spectrum or the line positions of transitions in this region. Emission from the first overtone was observed previously at medium resolution (0:2 cm1 ) by Tuckett et al. [15] with a SISAM spectrometer. Both the perpendicular B-type transitions and parallel A-type transitions were observed by Tuckett et al. with the predominant band being the A-type. The A/B hybrid nature of the transition is a result of the O–H stretching motion being in the a; b plane of the molecule. The stretching thus generates an alternating dipole along the a and b axes. Hybrid bands have not previously been observed in either m1 , m2 , or m3 . Only B-type transitions were reported by Yamada et al. [11] for the m1 fundamental, whereas only A-type transitions were observed by Burkholder [13] for m2 and m3 . Tuckett et al. [15] observed the Q branches of the DK ¼ 0 transitions as bandheads with the intensity rapidly falling off with N 0 . Asymmetry splitting was observed in only the Ka0 ¼ 1 transitions and the spin-splitting of the subbands was not resolved. Tuckett et al. fit their assignments of their 0:2 cm1 -resolution spectrum to a 7 term Hamiltonian and reported values for the upper state rotational constants A, B, C, DK , and DN , locating the origin at 6646:46 0:15 cm1 . The lack of observed perturbations by Tuckett et al. in the overtone spectrum is interesting given that Barclay et al. [16,17] predict the 2m1 to be in Fermi resonance with the (m2 þ 5m3 ). Using the three-dimensional potential energy surface of Walch and Duchovic. [18] Barclay et al. calculated the vibrational wavefunctions of HO2 in an attempt to identify low-lying Fermi-resonant pairs. The origin of the 2m1 and (m2 þ 5m3 ) were predicted by Barclay et al. to be only 7:3 cm1 apart. The Fermi coupling between these two states was predicted to be significant with the predicted coupling matrix element ¼ 3:2 0:2 cm1 . The coupling is strong enough that Barclay et al. suggest that ð0; 1; 5Þ ! ð0; 0; 0Þ transitions might be observed in the 2m1 spectrum. In this work the high-resolution infrared spectrum of HO2 is measured from 6603.2 to 6685:5 cm1 by using diode laser frequency modulation spectroscopy. The first overtone of the O–H stretch of HO2 is observed and assigned at a much higher resolution (0:0003 cm1 ) than previously reported. The spectrum is observed to be an A/B hybrid band with both A-type and B-type transitions. The spectrum is found to be heavily perturbed. These perturbations make accurate assignment of upper state rotational constants difficult. A ‘‘best’’ fit to the assigned spectrum is presented fitting 390 transitions of the Ka0 ¼ 0–3, with N 0 up to 16. The rotational constants obtained from this ‘‘best’’ fit to the high-resolution spectrum are in significant disagreement with those reported by Tuckett et al. [15] on the basis of
their lower-resolution spectrum. The origin in the present work is found about 4:6 cm1 higher in energy than reported by Tuckett et al. The high-resolution assignment presented here should allow the temperature dependence of absorption cross-sections to be more accurately predicted. This should make the monitoring of HO2 by using the 2m1 transition more applicable to sensing in a variety of environments.
2. Experiment The pulse photolysis/long path absorption method used in this experiment is similar to that employed in previous experiments [4,19]. The infrared spectrometer uses a narrow linewidth (<300 kHz) cw tunable commercial infrared diode laser. The diode laser can be scanned continuously by 0:0003 cm1 (on average) frequency steps at the highest resolution for slightly more than 1 cm1 (30 GHz). The tuning range is limited by the scanning PZT of the laser head. Therefore the recorded HO2 spectrum is taken as a series of 30 GHz scans. Two beam splitters are used to separate the laser output into three separate beam paths. The first beam splitter is used to place part of the output through a 150 MHz stable etalon. The diode laser has a considerable non-linearity in the frequency step-size; the step-size can nearly double in magnitude across the 30 GHz tuning range. The fixed etalon is used as a frequency marker and allows the removal of the non-linearity found in the diode laser frequency step-size while scanning the laser at high resolution. The etalon is scanned with the laser frequency fixed to insure single mode laser operation before and after each wavelength scan. The second beam splitter is used to direct part of the laser output through a reference gas cell. The cell has a path length of 85 cm and is filled with 8–9 Torr of NH3 . The high-resolution spectrum of NH3 from 6400 to 6600 cm1 has previously been observed by Fourier transform spectrometry to an accuracy of 0:0005 cm1 [20]. The spectrum of NH3 is obtained simultaneously with the HO2 spectrum and is used to calibrate the absolute frequency of the HO2 spectrum. After the calibration procedure the transitions in the reference NH3 spectrum have a standard deviation of 0:0027 cm1 to those previously observed by Fourier transform spectrometry. The infrared (IR) laser output is passed 20 times through a Herriott-type flow cell [19]. The IR probe beam enters into the 1.4 m long flow cell through a CaF2 window and then enters the multipass cavity off axis through a notch in the back Herriott mirror and is multipassed through the flow cell. After exiting through a notch in the front Herriott mirror and passing through another CaF2 window, the probe beam is passed through a band pass filter and then focused onto a
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detector. The UV photolysis beam passes through the center of the front Herriott mirror and travels on axis through the quartz flow cell before passing through the center of the back Herriott mirror and exiting the cell. The total effective length of the region where the IR probe beam and UV photolysis beam overlap is 20 m. To increase the signal to noise ratio of the observed spectrum, two-tone frequency modulation of the infrared laser is employed. The HO2 is formed from the reaction of O2 with CH2 OH. Cl is generated by photolysis at 355 nm of Cl2 and CH2 OH is subsequently generated by the reaction of Cl with methanol. The CH2 OH radical then reacts with O2 to produce the HO2 radical and formaldehyde. This reaction produces a 100% yield of HO2 as a product [21]. The formation of HO2 by this method avoids the formation of OH that complicated the spectrum recorded by Tuckett et al. [15]. The overtones of formaldehyde [22] and methanol [23] are not expected to contribute in this region. Typical gas concentrations are ½O2 ¼ 1:2 1017 cm3 , ½Cl2 ¼ 3:1 1015 cm3 , and ½CH3 OH ¼ 5:5 1015 cm3 . Helium is added to a total density of 5:18 1017 cm3 .
3. Analysis The observed spectrum has 1000 lines between 6603.2 and 6685:5 cm1 . A significant number of lines was expected given that transitions may be split by both asymmetry doubling and spin-rotation interaction. This splitting can lead to a maximum of 12 branches for Atype transitions and 24 branches for B-type transitions. The ground state rotational constants obtained by Fink and Ramsay can be used to calculate the ground state combination differences of HO2 . A computer program was then used to search through the spectrum for pairs of observed lines whose difference in energy (frequency) matched one of the ground state combination differences for the P and R of a common Ka0 value for the A-type transitions. The threshold for accepting a line pair as matching a ground state combination difference was set to (observed frequency difference – ground state combination difference) < 0:0120 cm1 . Pairs of lines that matched a ground state combination difference were then assigned the appropriate quantum numbers. These transitions could then be calculated by using the upper state constants of Tuckett et al. [15] and a modified version of the asymmetric top spectral fitting program of Sears [24] that utilizes the Watson A-reduced Hamiltonian with spin-rotation interaction. The residuals (observed – predicted) were then plotted against the N 0 quantum number to identify Ka0 series. The search was done starting with Ka0 ¼ 0 R and P transitions, but no clear series could be identified in the residuals plot. However, this particular series could be identified
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without the search program, as these transitions are some of the most intense in the observed spectrum. The upper state constants of Tuckett et al. [15] were then modified by doing a preliminary fit of the upper state that used the observed Ka0 ¼ 0 R, Q, and P branches. The most notable change is a 4:6 cm1 shift in the origin and a significant increase in the DN constant. These new preliminary upper state constants were used in the searching program to identify the R and P branches of a common Ka0 . Fig. 1 shows an example of a scatter plot that results from the searching program. The residuals calculated for lines matching the R and P Ka0 ¼ 1 NK0 0 ;N 0 ðK 0 þ1Þ F1 ground state combination difa a ferences are seen in the plot. While there are lots of line pairs that may coincidentally match the Ka0 ¼ 1 R and the P ground state combination difference in this spectral region the branches are clearly identifiable up to N 0 ¼ 14 in the scatter plot. Once the A-type transitions were identified by this program, B-type transitions could also be easily located by using ground state combination differences. Using this approach the A-type bands with Ka0 ¼ 0–3, N 0 6 16 and the B-type bands with Ka0 ¼ 0; 1 N 0 6 15 could be identified in the HO2 spectrum. In most cases an assigned transition for the Ka0 ¼ 0 or 1 series will fit at least 3 different ground state combination differences and in many cases more. A total of 491 transitions have been identified in the observed spectrum. These transitions have 554 unique ground state combination differences with a standard deviation of 0:0059 cm1 from those predicted by using the ground state constants of Fink and Ramsay.
Fig. 1. Residuals for observed transitions to a prediction of the transition frequency calculated by using the Watson A-reduced Hamiltonian versus the N 0 value. These residuals are for pairs of observed lines that have the correct ground state energy difference for transitions involving the R and P branch of Ka0 ¼ 1 NK0 0 N 0 ðK 0 þ1Þ F1 . The line pairs a a were found by a spectral searching program and fit by using preliminary upper state constants as described in the text. The ‘‘correct’’ Ka0 ¼ 1 series can easily be identified from the plot.
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Fig. 2. High-resolution spectrum of HO2 m1 first overtone showing the portions of the A-type band that was previously used to monitor HO2 for chemical kinetics [4,5,7]. The asymmetry splitting of the q P1 (11) is resolvable with the + and ) parity components labeled. The spin-rotation spitting is not resolvable for q P1 (11) or q P2 (9).
Fig. 2 shows a portion of the A-type transitions in the spectrum. The transition at 6625:7773 cm1 has previously been used to monitor HO2 formation from gas phase chemical reactions [4,5,25]. The observed spectrum has many features similar to those observed previously in the spectrum of Tuckett et al. [15]. The intensity of the A-type Q branches decrease rapidly with increasing N 0 . Except for Ka0 ¼ 0, the asymmetry splitting is observed to increase with increasing N 0 but decrease rapidly with increasing Ka0 . Tuckett et al. [15] were unable to resolve the spin splitting, but this spin splitting is resolvable at the higher resolution of the present study. The spin splitting of Ka0 ¼ 0 are not resolved, while the Ka0 ¼ 1, 2, and 3 exhibit spin splitting. The spin splitting in these series tends to decrease as N 0 increases. Fig. 3 shows an example of the B-type Ka0 series in the
spectrum. Both spin states of the R Q0 (N) are clearly observable in the spectrum. The Q branches with different Ka0 are widely spaced for the B-type transitions, so that only those transitions corresponding to Ka0 ¼ 0 and 1 have been identified. The searching program calculated the transitions in the spectrum using a Watson A-reduced Hamiltonian for an asymmetric-top that included spin-rotation and centrifugal distortion terms previously used by Fink and Ramsay [14]. Additional Hamiltonian terms are needed to adequately describe the energy levels of the overtone that are affected by Fermi resonance with the (m2 þ 5m3 ) levels. The lack of foreknowledge of the (m2 þ 5m3 ) upper state makes such a fit difficult. There are a large number of low intensity transitions observed in this spectral region, but there has yet to be an assignment of any of these transitions to the ðm2 þ 5m3 ). Generally, if the mixing of two upper state energy levels is locally confined to only a small number of N 0 states of a few Ka0 series then it is possible to produce an adequate fit to a Hamiltonian by just giving zero weight to those transitions that experience perturbation. Unfortunately, all the Ka0 series in the present spectrum appear to have some type of perturbation. Nonetheless, by leaving out both the Ka0 ¼ 1 NK0 a0 ;N 0 Ka0 F1 and F2 and the Ka0 ¼ 2 NK0 a0 ;N 0 Ka0 F1 and F2 , N 0 > 6 asymmetry branches, a final fit could be made that produces a semi-quantitative description of the observed spectrum. In total 390 transitions were used in the final fit with an average residual of 0:0046 cm1 . The resulting parameters from such a fit are of uncertain physical significance because several of the resulting parameters are of an unrealistically large magnitude. The resulting parameters are listed in Table 1. All 491 assigned transitions and their residuals to the fit are listed in the supplementary material.
4. Discussion
Fig. 3. High-resolution spectrum of HO2 m1 overtone showing both spin components of the R Q0 branch of the B-type band. Several transitions of the A-type band can also be seen in the spectrum.
The electronic band of HO2 A~2 A0 ! X~ 2 A00 ð0; 0; 0Þ ! ð0; 0; 0Þ was also observed and assigned by Tuckett et al. [15] using the same apparatus that observed the overtone band. This band was observed again by Fink and Ramsay [14] at much higher resolution and the assignments of Tuckett et al. were found to be essentially correct. The main deviation between the two studies was that the transitions observed by Tuckett et al. were found by Fink and Ramsay to be systematically lower by about 0:2 cm1 . The origin of the overtone band reported here is found to be different than Tuckett et al. by 4:6 cm1 . The origin found here is similar to the origin determined by Becker et al. at low resolution (6653 10 cm1 ) [26]. Since there are significant changes in many of the upper state centrifugal distortion terms, the deviation between the assignments presented here and assignments of Tuckett et al. are not simply a
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Table 1 Molecular constants of HO2 in MHz. Constant
2m1 (This work)
2m1 Tuckett et al. [15]
mo A B C DN DNK DK dN dK UN UNK UKN UK LK 108 eaa ebb ecc 1=2ðeabþ eba Þ DSK DSNK dSN USK
6651:1876ð38Þ cm1 566 706(50) 33 646(114) 31 501(62) 0.867(36) )12.18(84) 186.0(74) )0.196(11) 45(11) )0.00097(85) 0.0534(38) )0.465(70) 0.099 14(18)a )0.0001387(18)a )44 861(84) )356(84) )60(130)
6646:59ð15Þ cm1 545 982(330) 33 486(27) 31 676(27) 0.156(45) 3.60a 95.9(18)
a
83(36) )20(9) )9(9) )0.030 39(92)a
Ground state [27] 610 273.106(25) 33 517.733(15) 31 667.726(15) 0.116 865(10) 3.445 1(24) 123.572 4(35) 0.006 149 9(26) 1.977 9(75) 0.00001936(44) 0.00106(13) 0.099 14(18) )0.0001387(18) )49 572.006(113) )422.934(84) 8.748(84) )194.39(36) 23.664(21) 0.291(22) 0.159(12) )0.030 39(92)
Held fixed to the ground state value.
systematic offset. The previous assignment by Tuckett et al. [15] (Ka0 ¼ 0–4, N 00 up to 21) could be fit to a simple 7 term Hamiltonian. Given the perturbations expected in the spectrum the simplicity of the previous fit is somewhat surprising. As stated earlier the upper state constants of Tuckett et al. could not be used to identify the initial Ka0 ¼ 0 series observed at high resolution. The Ka0 series reported here are found to deviate somewhat even from the ‘‘best’’ fit to a much larger Hamiltonian than that used by Tuckett et al. As seen in Table 1 several resulting Hamiltonian parameters differ significantly from those determined by Tuckett et al. However, many of the qualitative features of the band described by Tuckett et al. do agree well with the observed highresolution spectrum. These features include the overtone band having significant intensity in both the A-type and the B-type transitions, the relative weakness of the Atype Q branches, and the Ka0 ¼ 1 branches being clearly asymmetry-split at low N 0 . It is possible that the more intense OH transitions in this region obscured too much of the spectrum observed by Tuckett et al. for an adequate assignment. Fink and Ramsay [14] have also speculated that the spectra measured by Tuckett et al. using a SISAM spectrometer was affected by instrumental artifacts. Given the lack of agreement between Tuckett et al. and this study for the band origin, rotational constants and amount of perturbations it can be concluded that the assignment of the overtone band reported here does not agree quantitatively with the previous assignment. A standard Watson A-reduced Hamiltonian for an asymmetric rotor is not adequate to reproduce the ob-
Fig. 4. Residuals for the Ka0 ¼ 1 NK0 a0 N 0 Ka0 F1 R branch as a result of the ‘‘best’’ fit to the HO2 m1 overtone spectrum. The series deviates significantly from the ‘‘best’’ fit to the spectrum and was given a weight of zero in the final fit.
served spectrum for the overtone. Fig. 4 shows the residuals for the R branch of the Ka0 ¼ 1 NK0 a0 ;N 0 Ka0 F1 that was left out of the ‘‘best’’ fit. It can be seen that even the ‘‘best’’ fit of the assigned spectrum does a poor job of reproducing most of the observed transitions in this particular Ka0 series. The average deviation of the observed spectrum to the fit is significantly better than this one series. As seen in Table 1 several of the higher order rotational constants needed to fit the upper state are significantly larger than those needed to fit the ground state. The centrifugal distortion constants ðDN , DNK , dN , UN , UNK , UKN ) involving N are noticeably larger than
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the ground state or those obtained by Hirota et al. for the unperturbed m1 . The cause of such large N dependent distortion in the upper state could be due to these series walking into resonance with nearby energy levels from the ðm2 þ 5m3 ) as N increases. Since the degree of overlap is unlikely to be the same for each Ka0 series it is perhaps fortuitous that such a good qualitative fit can be made at all. A more accurate representation of the overtone spectrum would require a Hamiltonian that includes terms that account for the Fermi resonance as well as knowledge of the upper state energy levels of the ðm2 þ 5m3 ). Thus while the parameters presented here may have limited physical significance, they do at least provide a concise summary of the experimental data. None of the ground state fundamentals have been observed to have the hybrid appearance of the O–H stretching overtone. The reported absence of the A-type bands in the m1 is particularly interesting given that as seen in Fig. 3 both the A-type and B-type transitions have significant intensity in the overtone spectrum. Most of the assigned transitions in the Yamada et al. [11] paper are of the B-type Q branches. It appears that only those B-type R and P transitions that run through the spectral regions very near these Q branches were assigned. The A-type Q branches in the overtone are weak. The strongest transitions of the A-type should therefore be in these R- and P-branch regions of the m1 spectrum that were not assigned by either Yamada et al. [11] or Burkholder et al. [13] Given that both m1 and 2m1 transitions involve motion of the hydrogen in the a/b plane and that both band types are so clearly observed in the overtone spectrum it would be interesting to reinvestigate the m1 for these A-type bands.
5. Conclusion The high-resolution infrared spectrum of HO2 has been observed from 6603.2 to 6685:5 cm1 by using diode laser frequency-modulation spectroscopy. The first overtone of the O–H stretch of HO2 has been observed and assigned for Ka0 ¼ 0–3, N 0 6 16. Both A-type and B-type transitions are observed in the spectrum. The spectrum is found to fit poorly to the Hamiltonian used in previous studies to fit the ground state and m1 fundamental. This is most likely due to the Fermi resonance predicted to exist between the overtone and the ðm2 þ 5m3 ). Given the degree of perturbation in the spectrum it seems likely that some weak ð0; 1; 5Þ ð0; 0; 0Þ transitions are indeed in the recorded spectrum. If these transitions can be identified, the degree of mixing between the two states may be able to be determined. The high-resolution assignment presented here will allow for the obtainment of accurate absolute number densities of the HO2 transitions at various temperatures. This should make the monitoring of HO2
by using the 2m1 more useful in remote sensing and kinetics applications.
Acknowledgments J.D.D. would to thank Prof. Robert F. Curl for useful discussions on this manuscript. This work is supported by the Division of Chemical Sciences, Geosciences, and Biosciences, the Office of Basic Energy Sciences, the US Department of Energy. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contact DE-AC0494-AL85000.
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