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Diode laser spectroscopy of the ν3 vibration of the HO2 radical
JOURNAL
OF MOLECULAR
SPECTROSCOPY
150,527-534 ( 1991)
Diode Laser Spectroscopy of the
v3
Vibration of the HO2 Radical
D. D. NELSON, JR., AND M. S. ZAHNISER Aerodyne Research, Inc., 45 Manning Road, Billerica, Massachusetts 01821
The infrared spectrum of the vj vibration of the hydroperoxyl radical has been examined using a tunable lead salt diode laser. Fifty-nine transitions were observed in the wavenumber interval between 1040 and 1120 cm-‘. This work extends the earlier measurements of Johns, McKellar, and Riggin to significantly higher rotational states and results in an improved set of spectroscopic constants which should predict all atmospherically relevant HO2 transition frequencies with a precision approaching a thousandth of a cm-‘. The incorporation of these line positions in atmospheric transmission models using HITRAN or ATMOS data bases will be useful in remote sensing measurements of HO*. 0 1991 Academic Press, hc. 1. INTRODUCTION
The HOz radical is an important trace species in the Earth’s atmosphere. HO* is the most abundant form of odd hydrogen in the troposphere and in most of the stratosphere (I). It is also a key species in photochemical ozone formation in the troposphere, and in catalytic ozone destruction in the stratosphere (2). It is thus important to spectroscopically characterize HO2 both to determine its atmospheric concentrations and to study its chemical properties in laboratory environments. The near UV spectrum of HOz is a broad continuum due to predissociation and hence has not been widely used for HOz detection. Infrared absorption spectroscopy of HOz , provides one promising approach to selective, sensitive detection of this species. The IR spectroscopy of HO2 has been previously studied by several groups (3-5) which have provided accurate high resolution absorption measurements in each of the three fundamental vibrational bands. However, in our previous studies of the absolute infrared absorption strengths of these bands (6, 7)) we noted sizeable discrepancies (as large as 0.2 cm-‘) between experimentally measured v3 line positions and those calculated from the best available molecular constants (8). These “discrepancies” begin for transitions with N” 2 10, where N” is the rotational quantum number for the lower vibrational state. Since only lines with N” G 7 were observed in Ref. (3) these differences do not imply any errors in the precise measurements of Johns et al. (3), but rather reveal the limited accuracy obtained by extrapolating the earlier low N” measurements to high N” transitions. In this work, we report the measurement of line positions for 59 HO2 transitions in the vgband with 6 < N” < 22. The Boltzmann distribution for HO:! peaks at N” E 10 at 300 K. We have also performed a fit to our data combined with those of Johns et al. (3) to determine an improved set of HOz molecular constants which can accurately (CT= 0.00 1 cm-’ ) predict all of the u3 HO2 transitions out of rotational states with significant populations at room temperature, i.e.. for N” < 30.
527
0022-2852/91 $3.00 Copyright 0
1991 by Academic Press. Inc.
All rights of reproduction in any form reserved.
528
NELSON
AND ZAHNISER
2. EXPERIMENTAL
PROCEDURE
The methods used to generate HO2 radicals and obtain their infrared spectra are presented in detail elsewhere (6, 7). In this section, we provide an overview of the experimental approach and the details of the frequency measurement procedure. The HOz radicals are created in a discharge flow system by the reaction F + H202 + HF + H02.
(1)
The F atoms are generated in a microwave discharge of F2 in helium. The pressure in the flow tube is -0.5 Torr and the most abundant species in the flow is the helium buffer gas. The HO* radicals flow across a multipass absorption cell where they are probed by an infrared diode laser beam. The absorption path length is augmented using a 40-pass White-type cell giving a total path through the HOz flow region of approximately 200 cm. The IR radiation is provided by a lead salt diode laser cooled to -20 K by a closed cycle liquid helium refrigerator. The diode laser output is passed through a monochromator for mode selection and detected with a liquid nitrogen cooled HgCdTe detector. The widths of the observed transition are generally Doppler limited at 0.0023 cm-‘. The infrared absorption signals were acquired in slow scans ( - 1 min) while the diode laser was frequency modulated via the diode current. The modulation frequency was typically 1 kHz. A representative absorption spectrum is displayed in Fig. 1. The output of the photodetector was directed to a lock-in amplifier for second harmonic detection. The output of the lock-in amplifier was in turn monitored by an X-Y recorder whose x-axis was driven by the laser current and hence was roughly proportional to the laser frequency. For each spectral region studied, three channels of data were acquired. These were the HOz absorption spectrum, a reference calibration gas absorption spectrum, and an &talon scan providing wavenumber markers. The three channels were acquired sequentially in separate scans using one photodetector. This procedure was tested for reliability by repeat scanning of a single channel and found to be accurate to better than 0.0005 cm-‘. The reference gases used for wavenumber calibration were OCS ( 9) and NH3 ( 10). The Ctalon wavenumber markers were provided by the presence of the White-type cell. Weak overlap of adjacent reflection spots on the White cell mirrors leads to some photons traversing the cell using 4 fewer (or more) passes than normal. This implies a characteristic fringe spacing of l/4 d (0.0050 cm-’ in this case), where d is the distance between the mirrors (50 cm). Wavenumber measurements were made by referencing HOz absorptions to OCS absorptions. This was accomplished by counting the number of &talon fringes separating the two transitions and linearly interpolating fractional fringes. The &talon free spectral range was calibrated by measuring the relative wavenumbers of several pairs of OCS reference lines. The accuracy of the measurements is believed to approach 0.00 1 cm-’ . The most likely source of systematic error in these measurements is the potential presence of small laser mode hops which might occur between the HO* absorption and the OCS absorption. To minimize the possibility of this affecting our results, many of the HO2 transitions were measured using two separate OCS reference lines, one to higher wavenumber and the other to lower wavenumber. Of 62 measured lines with
529
SPECTROSCOPY OF THE u3 VIBRATION OF HO2
cm-’
0.10
3
2
I
I
I I
,
t--l*
1
o
d?
g
K”a
(N”=13)
I 1
I
JO I
I
1067.0
1067.5
WAVENUMBER
(cm- ‘)
FIG. 1. HO2 v3band second harmonic spectra. The strongest line in the OCS reference scan is at 1067.0689 cm-‘, and the ttalon fringe spacing is 0.0050 cm-‘. The lower trace shows the diode laser intensity (1,) versus laser current.
good S/N, 59 are included in Table I. One group of three lines was rejected because its measured wavenumber differed from that predicted in the final least-squares fit by 0.007 cm-’ which is well outside the uncertainty of the fit. We believe that this discrepancy is due to a small laser mode hop between the HOz peaks and the OCS reference peak. Unfortunately, the position of this group of lines could only be referred to a single OCS line. 3. RESULTS AND DISCUSSION
The HOz radical is a near prolate top (K = -0.993 ) . The u3 vibration at 1097 cm-’ has been assigned to the O-O stretch (II, 12) and therefore gives rise to a parallel vibrational transition. The spectrum is more complex than that of a simple asymmetric top due to the unpaired electron in the hydroperoxyl radical which gives rise to a doubling of the number of states and a large spin-rotation interaction. To precisely calculate the HO* spectrum we developed a program based on the Hamiltonian of Bowater et al. (1.3). We used the A-reduced form of this Hamiltonian as described by Brown and Sears (14). Since the total angular momentum, J, is a good quantum label, each Jlevel of the upper and lower vibrational state was separately diagonalized
530
NELSON
AND
ZAHNISER
TABLE I Observed
Transitions
of HO2 Observed
J’
N’
20.5 20 20.5 20 19.5 20 19.5 20 20.5 20 20.5 20 19.5 20 20.5 20 19.5 20 18.5 19 19.519 18.5 19 19.519 19.519 18.5 19 12.5 12 12.5 12 11.5 12 12.5 12 12.5 12 12.5 12 11.5 12 11.5 12 11.5 12 12.5 12 12.5 12 11.5 12 10.5 10 9.5 10 9.5 10 9.5 10 10.5 10 10.5 10 9.5 10 a.5 8 7.5 8 a.5 8 8.5 8 8.5 a 8.5 8 7.5 8 7.5 8 7.5 8 1.5 8 8.5 8 8.5 8 7.5 8 6.5 6 6.5
6
6.5 5.5 10.5 10.5 9.5 9.5 10.5 10.5 10.5 9.5
6 6 10 10 10 lo 10 10 10 10
K,‘&
4 17 1 19 1 19 4 lb 3 17 3 18 2 18 2 19 2 19 2 17 218 0 19 019 119 1 19 5 8 4 9 5 7 3 9 1 11 2 11 2 10 2 11 0 12 0 12 1 12 1 12 2 8 2 8 2 9 0 10 0 10 1 10 1 10 4 4 5 3 3 6 1 7 2 6 2 7 3 5 2 6 2 7 0 a 0 8 1 8 1 8 1 5 2 4 2 5 1 5 1 10 3 7 2 9 2 a 2 9 2 8 0 10 0 10
'
,,"
21.5 21 21.5 21 20.5 21 20.5 21 21.5 21 21.5 21 20.5 21 21.5 21 20.5 21 19.520 20.5 20 19.5 20 20.5 20 20.5 20 19.5 20 13.5 13 13.5 13 12.5 13 13.5 13 13.5 13 13.5 13 12.5 13 12.5 13 12.5 13 13.5 13 13.5 13 12.5 13 11.5 11 10.5 11 10.5 11 10.5 11 11.5 11 11.5 11 10.5 11 9.5 9 8.5 9 9.5 9 9.5 9 9.5 9 9.5 9 8.5 9 8.5 9 8.5 9 a.5 9 9.5 9.5
8.5 7.5 7.5 7.5
9 9
9 7 7 7 6.5 7 9.5 9 9.5 9 a.5 9 a.5 9 9.5 9 9.5 9 9.5 9 8.5 9
KanKcw
Wavenumber (O-C) x IO3
4 18 1 20 1 20 4 17 3 18 3 19 2 19 2 20 2 20 218 2 19 0 20 0 20 1 20 1 20 5 9 4 10 5 8 3 10 1 12 2 12 2 11 2 12 0 13 0 13 1 13 1 13 2 9 2 9 2 10 0 11 0 11 1 11 1 11 4 5 5 4 3 7 1 8 2 7 2 8 3 6 2 7 2 8 0 9
1045.992
0
9
1 1 1 2 2 1 1 3 2 2 2 2 0 0
9 9 6
5 6
6 9 6
a 7 8 7 9 9
1046.116 1046.120 1046.146 1046.252 1046.260 1046.365 1046.505 1046.542 1049.064 1049.176 1049.462 1049.462 1049.810 1049.820 1066.293 1066.671 1066.744 1066.968 1066.917
1067.196 1067.230 1067.278 1067.402 1067.408 1067.683 1067.702 1072.087 1072.192 1072.224 1072.319 1072.326 1072.562 1072.587 1076.351 1076.653 1076.682 1076.a30 1076.914 1076.930 1076.994 1077.063 1077.078 1077.141 1077.147 1077.337 1077.375 1081.603 1081.623 1081.632 1081.659 1117.541 1117.616 1117.741 1117.769 1117.779 1117.805 1117.886 1117.895
1
0 -1 0 -2 0 0 1 0 2 0 1 -1 -1 0 0 0 -1 1 1 1 1 2 -1 1 1 1 -1 -2 1 -1 1 1 -1 -2 0 0 0 -1 -1 -1 0 -1 -1 0 -1 -1 -2 -1 0 1 0 0 -1 0 0 0 -2 -1
SPECTROSCOPY
OF THE
u, VIBRATION
OF HO2
531
including K levels up to the lesser of KobS+ 4 and K = J + 4. This program was extensively checked against published spectra and spectroscopic constants to verify its accuracy. The transitions observed in this work have N” = 7-21 with K < 6 and are listed in Table I. All observed transitions were either Fr f FI or FZ + Fz; no satellite transitions were measured. Transitions with N” < 10 were observed at positions corresponding closely to predictions based on the spectroscopic constants of McKellar (8), and hence were trivial to assign based on the earlier work. However, for lines with N” > 10 discrepancies between the observed and calculated spectra became noticeable and were quite large (0.2 cm-’ ) for N” = 20 and 2 1. Assignments of the new transitions were made by iteratively improving the spectroscopic constants with the inclusion of higher N” lines (whose assignment was obvious) in each successive fit of the data. The nonlinear least-squares fits were carried out by coupling the spectroscopic program described above to a general nonlinear least-squares fitting program found in the Starpac library ( 15). Fourteen upper state constants were determined in the final lit while four upper state constants were fixed at their ground state values. The constants which were fixed in the final fit had been shown to be insignificantly determined in preliminary fits to our data. The ground state molecular constants were fixed at the values of Charo and De Lucia (16). The data set consisted ofthe 59 transitions observed in this work, and of 71 transitions calculated from the spectroscopic constants of McKellar (8). These transition wavenumbers are the zero field values for the transitions actually studied by Johns et al. (3) which were observed using laser magnetic resonance at Zeeman shifted frequencies. All transitions were given equal weight. The resulting spectroscopic constants are given in Table II along with 95% confidence limits. The observed minus calculated wavenumbers for the transitions observed in this work are given in Table I. The standard deviation of the fit is G = 0.0007 cm-’ which is consistent with our expected measurement precision. As one would expect, the values of the lowest order constants are very similar to those reported by McKellar (8) with significant fractional differences occurring only in the higher order constants. In most cases the constants determined in this work agree with the earlier values within the combined reported uncertainties. However, the higher order constants are more precisely determined in this work, leading to a greatly improved ability to predict the positions of HOz transitions with N” > 10. In particular, the spectroscopic constants which describe the effect of increasing N (A, and 6N) are much better determined in the present work. Tables of spectral positions and absolute intensities as a function of temperature for the entire u3 band are available upon request. The spectral position and relative intensity of each line is calculated from the spectroscopic constants of this work, taking full account of asymmetry and spin rotation effects. This relative intensity is converted to an absolute intensity by referencing each line to our earlier absolute band strength measurements (6, 7). It is worth noting that no anomalous relative intensities were observed in this work. The relative intensities observed were reliable to approximately f20%. The imprecision is due mainly to using second harmonic detection rather than integrated direct absorption. The calculated transition positions should be accurate to -0.001 cm-’ while the absolute intensities are uncertain to -30% due to uncer-
NELSON
532
AND ZAHNISER TABLE II
Molecular Parameters of HO2 (cm-’ ) Parmaters
I
stat&
Ground State?
v3
A
20.356 524
20.309 40 (16)
B
1.118 0340
1.105 538 (25)
C
1.056 3192
1.042 726 (24)
lo3 Ax
4.122
4.202
(20)
=
1097.625 1 (3)
%
104 Am
1.149
1.248
( 3)
lo6 AN
3.900
3.887
(14)
105 61(
6.73
5.6
(11)
10' 6N
2.05
2.46
(16)
106 Ha
3.23
4.4
(6)
108 Ha
3.51
c
1010 Iim
7.6
c
=aa
-1.653 524
-1.712 0
Ebb
-1.410 2
-1.436
(12)
=cc
2.87
3.1
(11)
102 104
1.294
c
lo4 ASK
7.692
8.0
106 Asm
4.21
c
10' 1 cab + Eba
(4)
(3)
aFrom Charo and De Lucia, Reference 16. bPres.nt results from a cabined fit to diode laser transitions reported in this work together with data of Johns et al.3 Uncertainties in parentheses are 95% confidence limits. CParameter fired at ground state value.
tainties in the band strength measurement (6). We also mention that since HO2 is very nearly a symmetric prolate top, the effects of asymmetry on the transition intensities are very small being of the order of 1% for strong transitions. We have therefore derived generalized Honl-London factors for this open shell species which provide relative rovibrational intensities in the limit of small asymmetry and small spin-rotation interaction. For HOz these analytic expressions are quite useful since they reproduce the exact intensity calculations to within - 10% for allowed transitions. The entire HOz u3 absorption band is calculated at T = 296 K and displayed in Fig. 2. This figure includes all HO> transitions with IV”d 25 and K” G 4. The absorbance is plotted for an HOz column density of 2 X 10 ls cm-’ and assuming Doppler limited linewidths for all transitions. Although this plot appears as a stick figure, the partial overlap of adjacent transitions is taken into account by calculating absorbances at
533
SPECTROSCOPY OF THE iq VIBRATION OF HO2
1
[HO,] = 10’ 3 cm- 3 pathlength
= 200 cm
T = 296 K
1040
1060
1060
1100
1120
1140
WAVENUMBER (cm- ‘) Rc;. 2. Stick figure of HO* vj vibrational band. Absorbance is plotted vs. wavenumber for an HO2 column density of 2 X 10 Is cm-’ at T = 296 K.
each 0.00 1 cm-’ interval. Hence, in the R branch there are several features which are “extra” strong and are ideal candidates for monitoring HOz . 4. SUMMARY AND CONCLUSIONS
In this work we have revisited the v3 infrared spectrum of the hydroperoxyl radical, focusing on transitions with moderate to high values of N”. We have determined an improved set of molecular constants for this vibrational state which allow the precise prediction of HO* line positions for all rotational states which may be thermally populated under atmospheric conditions. These results, when combined with our previous determination of the absolute bandstrength for the v3 band (6)) will allow these lines to be incorporated into standard spectral libraries such as HITRAN ( 10) and ATMOS ( 17) and may be useful in the remote spectroscopic detection of the HOz radical. ACKNOWLEDGMENT We thank the NASA Upper Atmospheric Research Program for the support of this work under Contract NASW 4339. RECEIVED:
June 17, 1991 REFERENCES
1. 2. 3. 4.
G. BRASSEUR AND S. SOLOMON,“Aeronomy of the Middle Atmosphere,” Reidel. Boston, 1984. H. S. JOHNSTON,Ann. Rev. Phys. Chem. 35,48 l-505 ( 1984). J. W. C. JOHNS,A. R. W. MCKELLAR,ANDM. RIGGIN, J. Chem. Phys. 68,3957-3966 ( 1978). K. NAGAI, Y. ENDO, AND E. HIROTA, J. Mol. Spectrosc. 89,520-527 ( 198 1). 5. C. YAMADA, Y. ENDO, AND E. HIROTA, J. Chem. Phys. 78,4379-4384 (1983).
534
NELSON AND ZAHNISER
6. M. S. ZAHNISERAND A. C. STANTON,J. Chem. Phys. 80,4951-4960 ( 1984). 7. M. S. ZAHNISER,K. E. MCCXJRDY, AND A. C. STANTON,J. Chem. Phys. 93, 1065-1070 (1989). 8. A. R. W. MCKELLAR,Faraday Discuss. Chem. Sot. 71,63-73 (1981). 9. J. S. WELLS,F. R. PETERSEN, AND A. G. MAKI, Appl. Opt. 18, 3567-3573 (1979), AND A. G. MAKI AND J. S. WELLS,private communication. 10. L. S. ROTHMAN,R. R. GAMACHE,A. GOLDMAN,L. R. BROWN,R. A. TOTH, H. M. PICKETT,R. L. POYNTER,J.-M. FLAUD,C. CAMY-PEYRET, A. BARBE,N. HUSSON,C. P. RINSLAND,AND M. A. H. SMITH,Appl. Opt. 26,4058-4097 (1987). Il. D. W. SMITHAND L. ANDREW&J. Chem. Phys. 60, 81-85 ( 1974) and referencestherein. 12. T. T. PAUKERTAND H. S. JOHNSTON,J. Chem. Phys. 56,2824-2838 ( 1972). 13. I. C. B~WATER,J. M. BROWN,AND A. CARRINGTON, Proc. R. Sot. London A 333,265-288 ( 1973). 14. J. M. BROWNAND T. J. SEARS,J. Mol. Spectrosc. 75, 111-133 (1979). IS. J. R. DONALDSONAND P. V. TYRON, “NBS TechnicalNote 1068-1,” Washington,DC, 1983. 16. A. CHAROAND F. C. DE LUCIA,J. Mol. Spectrosc. 94,426-436 ( 1982). 17. L. R. BROWN,C. B. FARMER,C. P. RINSLAND,AND R. A. TOTH, Appl. Opt. 26, 5154-5182 ( 1987).
OF MOLECULAR
SPECTROSCOPY
150,527-534 ( 1991)
Diode Laser Spectroscopy of the
v3
Vibration of the HO2 Radical
D. D. NELSON, JR., AND M. S. ZAHNISER Aerodyne Research, Inc., 45 Manning Road, Billerica, Massachusetts 01821
The infrared spectrum of the vj vibration of the hydroperoxyl radical has been examined using a tunable lead salt diode laser. Fifty-nine transitions were observed in the wavenumber interval between 1040 and 1120 cm-‘. This work extends the earlier measurements of Johns, McKellar, and Riggin to significantly higher rotational states and results in an improved set of spectroscopic constants which should predict all atmospherically relevant HO2 transition frequencies with a precision approaching a thousandth of a cm-‘. The incorporation of these line positions in atmospheric transmission models using HITRAN or ATMOS data bases will be useful in remote sensing measurements of HO*. 0 1991 Academic Press, hc. 1. INTRODUCTION
The HOz radical is an important trace species in the Earth’s atmosphere. HO* is the most abundant form of odd hydrogen in the troposphere and in most of the stratosphere (I). It is also a key species in photochemical ozone formation in the troposphere, and in catalytic ozone destruction in the stratosphere (2). It is thus important to spectroscopically characterize HO2 both to determine its atmospheric concentrations and to study its chemical properties in laboratory environments. The near UV spectrum of HOz is a broad continuum due to predissociation and hence has not been widely used for HOz detection. Infrared absorption spectroscopy of HOz , provides one promising approach to selective, sensitive detection of this species. The IR spectroscopy of HO2 has been previously studied by several groups (3-5) which have provided accurate high resolution absorption measurements in each of the three fundamental vibrational bands. However, in our previous studies of the absolute infrared absorption strengths of these bands (6, 7)) we noted sizeable discrepancies (as large as 0.2 cm-‘) between experimentally measured v3 line positions and those calculated from the best available molecular constants (8). These “discrepancies” begin for transitions with N” 2 10, where N” is the rotational quantum number for the lower vibrational state. Since only lines with N” G 7 were observed in Ref. (3) these differences do not imply any errors in the precise measurements of Johns et al. (3), but rather reveal the limited accuracy obtained by extrapolating the earlier low N” measurements to high N” transitions. In this work, we report the measurement of line positions for 59 HO2 transitions in the vgband with 6 < N” < 22. The Boltzmann distribution for HO:! peaks at N” E 10 at 300 K. We have also performed a fit to our data combined with those of Johns et al. (3) to determine an improved set of HOz molecular constants which can accurately (CT= 0.00 1 cm-’ ) predict all of the u3 HO2 transitions out of rotational states with significant populations at room temperature, i.e.. for N” < 30.
527
0022-2852/91 $3.00 Copyright 0
1991 by Academic Press. Inc.
All rights of reproduction in any form reserved.
528
NELSON
AND ZAHNISER
2. EXPERIMENTAL
PROCEDURE
The methods used to generate HO2 radicals and obtain their infrared spectra are presented in detail elsewhere (6, 7). In this section, we provide an overview of the experimental approach and the details of the frequency measurement procedure. The HOz radicals are created in a discharge flow system by the reaction F + H202 + HF + H02.
(1)
The F atoms are generated in a microwave discharge of F2 in helium. The pressure in the flow tube is -0.5 Torr and the most abundant species in the flow is the helium buffer gas. The HO* radicals flow across a multipass absorption cell where they are probed by an infrared diode laser beam. The absorption path length is augmented using a 40-pass White-type cell giving a total path through the HOz flow region of approximately 200 cm. The IR radiation is provided by a lead salt diode laser cooled to -20 K by a closed cycle liquid helium refrigerator. The diode laser output is passed through a monochromator for mode selection and detected with a liquid nitrogen cooled HgCdTe detector. The widths of the observed transition are generally Doppler limited at 0.0023 cm-‘. The infrared absorption signals were acquired in slow scans ( - 1 min) while the diode laser was frequency modulated via the diode current. The modulation frequency was typically 1 kHz. A representative absorption spectrum is displayed in Fig. 1. The output of the photodetector was directed to a lock-in amplifier for second harmonic detection. The output of the lock-in amplifier was in turn monitored by an X-Y recorder whose x-axis was driven by the laser current and hence was roughly proportional to the laser frequency. For each spectral region studied, three channels of data were acquired. These were the HOz absorption spectrum, a reference calibration gas absorption spectrum, and an &talon scan providing wavenumber markers. The three channels were acquired sequentially in separate scans using one photodetector. This procedure was tested for reliability by repeat scanning of a single channel and found to be accurate to better than 0.0005 cm-‘. The reference gases used for wavenumber calibration were OCS ( 9) and NH3 ( 10). The Ctalon wavenumber markers were provided by the presence of the White-type cell. Weak overlap of adjacent reflection spots on the White cell mirrors leads to some photons traversing the cell using 4 fewer (or more) passes than normal. This implies a characteristic fringe spacing of l/4 d (0.0050 cm-’ in this case), where d is the distance between the mirrors (50 cm). Wavenumber measurements were made by referencing HOz absorptions to OCS absorptions. This was accomplished by counting the number of &talon fringes separating the two transitions and linearly interpolating fractional fringes. The &talon free spectral range was calibrated by measuring the relative wavenumbers of several pairs of OCS reference lines. The accuracy of the measurements is believed to approach 0.00 1 cm-’ . The most likely source of systematic error in these measurements is the potential presence of small laser mode hops which might occur between the HO* absorption and the OCS absorption. To minimize the possibility of this affecting our results, many of the HO2 transitions were measured using two separate OCS reference lines, one to higher wavenumber and the other to lower wavenumber. Of 62 measured lines with
529
SPECTROSCOPY OF THE u3 VIBRATION OF HO2
cm-’
0.10
3
2
I
I
I I
,
t--l*
1
o
d?
g
K”a
(N”=13)
I 1
I
JO I
I
1067.0
1067.5
WAVENUMBER
(cm- ‘)
FIG. 1. HO2 v3band second harmonic spectra. The strongest line in the OCS reference scan is at 1067.0689 cm-‘, and the ttalon fringe spacing is 0.0050 cm-‘. The lower trace shows the diode laser intensity (1,) versus laser current.
good S/N, 59 are included in Table I. One group of three lines was rejected because its measured wavenumber differed from that predicted in the final least-squares fit by 0.007 cm-’ which is well outside the uncertainty of the fit. We believe that this discrepancy is due to a small laser mode hop between the HOz peaks and the OCS reference peak. Unfortunately, the position of this group of lines could only be referred to a single OCS line. 3. RESULTS AND DISCUSSION
The HOz radical is a near prolate top (K = -0.993 ) . The u3 vibration at 1097 cm-’ has been assigned to the O-O stretch (II, 12) and therefore gives rise to a parallel vibrational transition. The spectrum is more complex than that of a simple asymmetric top due to the unpaired electron in the hydroperoxyl radical which gives rise to a doubling of the number of states and a large spin-rotation interaction. To precisely calculate the HO* spectrum we developed a program based on the Hamiltonian of Bowater et al. (1.3). We used the A-reduced form of this Hamiltonian as described by Brown and Sears (14). Since the total angular momentum, J, is a good quantum label, each Jlevel of the upper and lower vibrational state was separately diagonalized
530
NELSON
AND
ZAHNISER
TABLE I Observed
Transitions
of HO2 Observed
J’
N’
20.5 20 20.5 20 19.5 20 19.5 20 20.5 20 20.5 20 19.5 20 20.5 20 19.5 20 18.5 19 19.519 18.5 19 19.519 19.519 18.5 19 12.5 12 12.5 12 11.5 12 12.5 12 12.5 12 12.5 12 11.5 12 11.5 12 11.5 12 12.5 12 12.5 12 11.5 12 10.5 10 9.5 10 9.5 10 9.5 10 10.5 10 10.5 10 9.5 10 a.5 8 7.5 8 a.5 8 8.5 8 8.5 a 8.5 8 7.5 8 7.5 8 7.5 8 1.5 8 8.5 8 8.5 8 7.5 8 6.5 6 6.5
6
6.5 5.5 10.5 10.5 9.5 9.5 10.5 10.5 10.5 9.5
6 6 10 10 10 lo 10 10 10 10
K,‘&
4 17 1 19 1 19 4 lb 3 17 3 18 2 18 2 19 2 19 2 17 218 0 19 019 119 1 19 5 8 4 9 5 7 3 9 1 11 2 11 2 10 2 11 0 12 0 12 1 12 1 12 2 8 2 8 2 9 0 10 0 10 1 10 1 10 4 4 5 3 3 6 1 7 2 6 2 7 3 5 2 6 2 7 0 a 0 8 1 8 1 8 1 5 2 4 2 5 1 5 1 10 3 7 2 9 2 a 2 9 2 8 0 10 0 10
'
,,"
21.5 21 21.5 21 20.5 21 20.5 21 21.5 21 21.5 21 20.5 21 21.5 21 20.5 21 19.520 20.5 20 19.5 20 20.5 20 20.5 20 19.5 20 13.5 13 13.5 13 12.5 13 13.5 13 13.5 13 13.5 13 12.5 13 12.5 13 12.5 13 13.5 13 13.5 13 12.5 13 11.5 11 10.5 11 10.5 11 10.5 11 11.5 11 11.5 11 10.5 11 9.5 9 8.5 9 9.5 9 9.5 9 9.5 9 9.5 9 8.5 9 8.5 9 8.5 9 a.5 9 9.5 9.5
8.5 7.5 7.5 7.5
9 9
9 7 7 7 6.5 7 9.5 9 9.5 9 a.5 9 a.5 9 9.5 9 9.5 9 9.5 9 8.5 9
KanKcw
Wavenumber (O-C) x IO3
4 18 1 20 1 20 4 17 3 18 3 19 2 19 2 20 2 20 218 2 19 0 20 0 20 1 20 1 20 5 9 4 10 5 8 3 10 1 12 2 12 2 11 2 12 0 13 0 13 1 13 1 13 2 9 2 9 2 10 0 11 0 11 1 11 1 11 4 5 5 4 3 7 1 8 2 7 2 8 3 6 2 7 2 8 0 9
1045.992
0
9
1 1 1 2 2 1 1 3 2 2 2 2 0 0
9 9 6
5 6
6 9 6
a 7 8 7 9 9
1046.116 1046.120 1046.146 1046.252 1046.260 1046.365 1046.505 1046.542 1049.064 1049.176 1049.462 1049.462 1049.810 1049.820 1066.293 1066.671 1066.744 1066.968 1066.917
1067.196 1067.230 1067.278 1067.402 1067.408 1067.683 1067.702 1072.087 1072.192 1072.224 1072.319 1072.326 1072.562 1072.587 1076.351 1076.653 1076.682 1076.a30 1076.914 1076.930 1076.994 1077.063 1077.078 1077.141 1077.147 1077.337 1077.375 1081.603 1081.623 1081.632 1081.659 1117.541 1117.616 1117.741 1117.769 1117.779 1117.805 1117.886 1117.895
1
0 -1 0 -2 0 0 1 0 2 0 1 -1 -1 0 0 0 -1 1 1 1 1 2 -1 1 1 1 -1 -2 1 -1 1 1 -1 -2 0 0 0 -1 -1 -1 0 -1 -1 0 -1 -1 -2 -1 0 1 0 0 -1 0 0 0 -2 -1
SPECTROSCOPY
OF THE
u, VIBRATION
OF HO2
531
including K levels up to the lesser of KobS+ 4 and K = J + 4. This program was extensively checked against published spectra and spectroscopic constants to verify its accuracy. The transitions observed in this work have N” = 7-21 with K < 6 and are listed in Table I. All observed transitions were either Fr f FI or FZ + Fz; no satellite transitions were measured. Transitions with N” < 10 were observed at positions corresponding closely to predictions based on the spectroscopic constants of McKellar (8), and hence were trivial to assign based on the earlier work. However, for lines with N” > 10 discrepancies between the observed and calculated spectra became noticeable and were quite large (0.2 cm-’ ) for N” = 20 and 2 1. Assignments of the new transitions were made by iteratively improving the spectroscopic constants with the inclusion of higher N” lines (whose assignment was obvious) in each successive fit of the data. The nonlinear least-squares fits were carried out by coupling the spectroscopic program described above to a general nonlinear least-squares fitting program found in the Starpac library ( 15). Fourteen upper state constants were determined in the final lit while four upper state constants were fixed at their ground state values. The constants which were fixed in the final fit had been shown to be insignificantly determined in preliminary fits to our data. The ground state molecular constants were fixed at the values of Charo and De Lucia (16). The data set consisted ofthe 59 transitions observed in this work, and of 71 transitions calculated from the spectroscopic constants of McKellar (8). These transition wavenumbers are the zero field values for the transitions actually studied by Johns et al. (3) which were observed using laser magnetic resonance at Zeeman shifted frequencies. All transitions were given equal weight. The resulting spectroscopic constants are given in Table II along with 95% confidence limits. The observed minus calculated wavenumbers for the transitions observed in this work are given in Table I. The standard deviation of the fit is G = 0.0007 cm-’ which is consistent with our expected measurement precision. As one would expect, the values of the lowest order constants are very similar to those reported by McKellar (8) with significant fractional differences occurring only in the higher order constants. In most cases the constants determined in this work agree with the earlier values within the combined reported uncertainties. However, the higher order constants are more precisely determined in this work, leading to a greatly improved ability to predict the positions of HOz transitions with N” > 10. In particular, the spectroscopic constants which describe the effect of increasing N (A, and 6N) are much better determined in the present work. Tables of spectral positions and absolute intensities as a function of temperature for the entire u3 band are available upon request. The spectral position and relative intensity of each line is calculated from the spectroscopic constants of this work, taking full account of asymmetry and spin rotation effects. This relative intensity is converted to an absolute intensity by referencing each line to our earlier absolute band strength measurements (6, 7). It is worth noting that no anomalous relative intensities were observed in this work. The relative intensities observed were reliable to approximately f20%. The imprecision is due mainly to using second harmonic detection rather than integrated direct absorption. The calculated transition positions should be accurate to -0.001 cm-’ while the absolute intensities are uncertain to -30% due to uncer-
NELSON
532
AND ZAHNISER TABLE II
Molecular Parameters of HO2 (cm-’ ) Parmaters
I
stat&
Ground State?
v3
A
20.356 524
20.309 40 (16)
B
1.118 0340
1.105 538 (25)
C
1.056 3192
1.042 726 (24)
lo3 Ax
4.122
4.202
(20)
=
1097.625 1 (3)
%
104 Am
1.149
1.248
( 3)
lo6 AN
3.900
3.887
(14)
105 61(
6.73
5.6
(11)
10' 6N
2.05
2.46
(16)
106 Ha
3.23
4.4
(6)
108 Ha
3.51
c
1010 Iim
7.6
c
=aa
-1.653 524
-1.712 0
Ebb
-1.410 2
-1.436
(12)
=cc
2.87
3.1
(11)
102 104
1.294
c
lo4 ASK
7.692
8.0
106 Asm
4.21
c
10' 1 cab + Eba
(4)
(3)
aFrom Charo and De Lucia, Reference 16. bPres.nt results from a cabined fit to diode laser transitions reported in this work together with data of Johns et al.3 Uncertainties in parentheses are 95% confidence limits. CParameter fired at ground state value.
tainties in the band strength measurement (6). We also mention that since HO2 is very nearly a symmetric prolate top, the effects of asymmetry on the transition intensities are very small being of the order of 1% for strong transitions. We have therefore derived generalized Honl-London factors for this open shell species which provide relative rovibrational intensities in the limit of small asymmetry and small spin-rotation interaction. For HOz these analytic expressions are quite useful since they reproduce the exact intensity calculations to within - 10% for allowed transitions. The entire HOz u3 absorption band is calculated at T = 296 K and displayed in Fig. 2. This figure includes all HO> transitions with IV”d 25 and K” G 4. The absorbance is plotted for an HOz column density of 2 X 10 ls cm-’ and assuming Doppler limited linewidths for all transitions. Although this plot appears as a stick figure, the partial overlap of adjacent transitions is taken into account by calculating absorbances at
533
SPECTROSCOPY OF THE iq VIBRATION OF HO2
1
[HO,] = 10’ 3 cm- 3 pathlength
= 200 cm
T = 296 K
1040
1060
1060
1100
1120
1140
WAVENUMBER (cm- ‘) Rc;. 2. Stick figure of HO* vj vibrational band. Absorbance is plotted vs. wavenumber for an HO2 column density of 2 X 10 Is cm-’ at T = 296 K.
each 0.00 1 cm-’ interval. Hence, in the R branch there are several features which are “extra” strong and are ideal candidates for monitoring HOz . 4. SUMMARY AND CONCLUSIONS
In this work we have revisited the v3 infrared spectrum of the hydroperoxyl radical, focusing on transitions with moderate to high values of N”. We have determined an improved set of molecular constants for this vibrational state which allow the precise prediction of HO* line positions for all rotational states which may be thermally populated under atmospheric conditions. These results, when combined with our previous determination of the absolute bandstrength for the v3 band (6)) will allow these lines to be incorporated into standard spectral libraries such as HITRAN ( 10) and ATMOS ( 17) and may be useful in the remote spectroscopic detection of the HOz radical. ACKNOWLEDGMENT We thank the NASA Upper Atmospheric Research Program for the support of this work under Contract NASW 4339. RECEIVED:
June 17, 1991 REFERENCES
1. 2. 3. 4.
G. BRASSEUR AND S. SOLOMON,“Aeronomy of the Middle Atmosphere,” Reidel. Boston, 1984. H. S. JOHNSTON,Ann. Rev. Phys. Chem. 35,48 l-505 ( 1984). J. W. C. JOHNS,A. R. W. MCKELLAR,ANDM. RIGGIN, J. Chem. Phys. 68,3957-3966 ( 1978). K. NAGAI, Y. ENDO, AND E. HIROTA, J. Mol. Spectrosc. 89,520-527 ( 198 1). 5. C. YAMADA, Y. ENDO, AND E. HIROTA, J. Chem. Phys. 78,4379-4384 (1983).
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NELSON AND ZAHNISER
6. M. S. ZAHNISERAND A. C. STANTON,J. Chem. Phys. 80,4951-4960 ( 1984). 7. M. S. ZAHNISER,K. E. MCCXJRDY, AND A. C. STANTON,J. Chem. Phys. 93, 1065-1070 (1989). 8. A. R. W. MCKELLAR,Faraday Discuss. Chem. Sot. 71,63-73 (1981). 9. J. S. WELLS,F. R. PETERSEN, AND A. G. MAKI, Appl. Opt. 18, 3567-3573 (1979), AND A. G. MAKI AND J. S. WELLS,private communication. 10. L. S. ROTHMAN,R. R. GAMACHE,A. GOLDMAN,L. R. BROWN,R. A. TOTH, H. M. PICKETT,R. L. POYNTER,J.-M. FLAUD,C. CAMY-PEYRET, A. BARBE,N. HUSSON,C. P. RINSLAND,AND M. A. H. SMITH,Appl. Opt. 26,4058-4097 (1987). Il. D. W. SMITHAND L. ANDREW&J. Chem. Phys. 60, 81-85 ( 1974) and referencestherein. 12. T. T. PAUKERTAND H. S. JOHNSTON,J. Chem. Phys. 56,2824-2838 ( 1972). 13. I. C. B~WATER,J. M. BROWN,AND A. CARRINGTON, Proc. R. Sot. London A 333,265-288 ( 1973). 14. J. M. BROWNAND T. J. SEARS,J. Mol. Spectrosc. 75, 111-133 (1979). IS. J. R. DONALDSONAND P. V. TYRON, “NBS TechnicalNote 1068-1,” Washington,DC, 1983. 16. A. CHAROAND F. C. DE LUCIA,J. Mol. Spectrosc. 94,426-436 ( 1982). 17. L. R. BROWN,C. B. FARMER,C. P. RINSLAND,AND R. A. TOTH, Appl. Opt. 26, 5154-5182 ( 1987).