Submillimeter-wave measurements of N2 and O2 pressure broadening for HO2 radical generated by Hg-photosensitized reaction

Journal of Quantitative Spectroscopy & Radiative Transfer 113 (2012) 279–285

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Submillimeter-wave measurements of N2 and O2 pressure broadening for HO2 radical generated by Hg-photosensitized reaction A. Mizoguchi n, T. Yagi 1, K. Kondo, T.O. Sato 2, H. Kanamori Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 Ohokayama, Meguro-ku, Tokyo 152-8550, Japan

a r t i c l e i n f o

abstract

Article history: Received 14 September 2011 Received in revised form 11 November 2011 Accepted 14 November 2011 Available online 25 November 2011

The N2 and O2 pressure broadening coefficients of the pure rotational transitions at 625.66 GHz (NKaKc ¼ 101 9–100 10, J¼10.5–10.5) and 649.70 GHz (NKaKc ¼102 9–92 8, J¼ 9.5– 8.5) in the vibronic ground state X2A0 of the perhydroxyl (HO2) radical have been determined by precise laboratory measurements. For the production of HO2, the mercury-photosensitized reaction of the H2 and O2 precursors was used to provide an optimum condition for measurement of the pressure broadening coefficient. The Superconducting Submillimeterwave Limb Emission Sounder (SMILES) was designed to monitor the volume mixing ratio of trace gases including HO2 in the Earth’s upper atmosphere using these transitions. The precise measurement of pressure broadening coefficient g in terms of the half width at half maximum is required in order to retrieve the atmospheric volume mixing ratio. In this work, g coefficients of the 625.66 GHz transition were determined for N2 and O2 at room temperature as g(N2)¼ 4.08570.049 MHz/Torr and g(O2)¼ 2.57870.047 MHz/Torr with 3s uncertainty. Similarly, the coefficients of the 649.70 GHz transition were determined as g(N2)¼3.48970.094 MHz/Torr and g(O2)¼ 2.61570.099 MHz/Torr. The air broadening coefficients for the 625.66 GHz and 649.70 GHz lines were estimated at g(air)¼3.7697 0.067 MHz and 3.29870.099 MHz respectively, where the uncertainty includes possible systematic errors. The newly determined coefficients are compared with previous results and we discuss the advantage of the mercury-photosensitized reaction for HO2 generation. In comparison with those of other singlet molecules, the pressure broadening coefficients of the HO2 radical are not much affected by the existence of an unpaired electron. & 2011 Elsevier Ltd. All rights reserved.

Keywords: Perhydroxyl radical Linewidths Pressure broadening Atmospheric spectra

1. Introduction The hydroperoxyl radical HO2, with an unpaired electron, plays an important role as an intermediate species in the Earth’s atmospheric chemistry. For example, HO2 is

n

Corresponding author. Fax: þ81 3 5734 2751. E-mail addresses: [email protected] (A. Mizoguchi), [email protected] (H. Kanamori). 1 Present address: The University of Tokyo, Bunkyo-ku, Tokyo, Japan. 2 Present address: Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Yokohama, Kanagawa 2268503, Japan. 0022-4073/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jqsrt.2011.11.009

one of the key radicals of the HOx cycle, by which ozone is depleted in the stratosphere [1], and HO2 plays a very important role in oxidation reactions for photochemical smog in the troposphere [2]. HO2 is produced by a reaction of OH and O3, OHþ O3-HO2 þO2. HO2 reacts with the O radical and changes into OH, HO2 þ O-OHþO2. Net O3 þO-O2 þO2

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In these ways, odd oxygen is converted into O2 molecules; that is, O3 decreases with an increase of intermediates such as HO2 and OH. Quantitative measurements of atmospheric trace gases including HO2 are expected to improve our knowledge of atmospheric chemical reactions. To satisfy such a desire, several missions have been planned to measure the global distributions of those important radicals in real time from satellites. For example, the EOS-MLS mission was planned by NASA to monitor these radicals in the submillimeter-wave region using the Earth Observing System Microwave Limb Sounder aboard the Aura spacecraft [3]. In Japan, the Superconducting Submillimeter-wave Limb Emission Sounder (SMILES) on the Japanese Experiment Module of the International Space Station was designed to observe atmospheric trace gases with errors of a few percent [4]. SMILES observed the middle atmosphere (8–90 km) from September 2009 to April 2010. SMILES offers the advantage of highly sensitive detection of the middle atmosphere using a low-noise receiver with a superconductor–insulator–superconductor (SIS) mixer in a mechanical cooler. For both missions, HO2 was listed as the target species. The pressure broadening coefficient g is one of the most important spectroscopic parameters in order to determine the altitude abundance of the species by applying a retrieval method to the observed spectral line profile [5]. It is expected that the g coefficient must be determined with a precision of a few percent to derive a reliable altitude distribution through the retrieval procedure. So far, the N2 and O2 pressure broadening coefficients of HO2 for several transitions in the submillimeter-wave region have been determined, as listed in Table 1. However, the measurement of the pressure dependence is not an easy task for radicals, because these are generated by chemical reactions that greatly depend on the pressure conditions. The previous works have been at some disadvantage to prepare the HO2 radical for measurement. Chance et al. produced the HO2 radical by discharge in He (1 Torr), O2

(1 Torr), Cl2 (10 mTorr), and CH3OH (10 mTorr) to measure the g coefficient of the NKaKc ¼ 132 12–121 11, J¼13.5–12.5 line at 2.4978 THz [6]. Although a large amount of He was necessary for maintaining a stable discharge, they needed to reduce the concentration of He as much as possible to avoid collisional effects, except from the buffer gases. The signal-to-noise (S/N) ratio of the spectrum was not good ( 4), because the discharge was unstable in the low concentration of He. As a result, the error in g was about 10% at one standard deviation (1s). Yamada measured the coefficients for the NKaKc ¼101 10–91 9, J¼9.5–8.5 line at 641.64 GHz and the NKaKc ¼102 9  92 8, J¼9.5–8.5 line at 649.70 GHz [7], of which the latter is the target of the SMILES mission. In that work, HO2 was produced outside a measuring cell by the external discharge method of O2 with allyl alcohol CH2 ¼CHCH2OH and then introduced into the cell. They succeeded in observing the spectra with good S/N ratio and determined the g coefficient for each line. It should be noted that their value for the 649.70 GHz line is fairly small compared with those for the other lines. They suggested that the coefficient for the 649.70 GHz line might involve a systematic error because the line happens to overlap with an unknown spectral peak, which may represent the precursor molecule or discharged derivatives. In this work, we introduce for the first time a mercuryphotosensitized reaction to produce a sufficient amount of HO2 to measure the g coefficient with less interference from other species. Furthermore, this method allows us to measure it under much higher pressure conditions than by other reaction methods. We report here the pressure broadening coefficients of the NKaKc ¼101 9–100 10, J¼ 10.5–10.5 line at 625.66 GHz, and the NKaKc ¼ 102 9–92 8, J¼ 9.5–8.5 line at 649.70 GHz. Both lines have been observed in the JEM/SMILES project. More accurate coefficients of pressure broadening by N2 and O2 at room temperature are determined by a convolution method, and then the air broadening coefficients are presented.

Table 1 Comparison of pressure broadening coefficients of HO2 measured in this study and previous ones. Transition N0 Ka0 Kc0  N00 Ka00 Kc00

J 0 2J 00

Frequency (GHz)

41 3–31 2

4.5–3.5

( ( 101 9–100 10c

10.5–10.5 (

101 10–91 9

9.5–8.5 (

c,d

102 9–92 8

9.5–8.5

( 132 a

12–121 11

13.5–12.5

c

g(N2)a (MHz/Torr) g(O2)a (MHz/Torr) g(air)b (MHz/Torr) Reference

4–3 5–4

265.7691 265.7702

32.2

4.049(65)

10–10 11–11

625.6603 625.6638

119.4

4.085(49)

2.578(47)

3.769(67)

This work

10–9 9–8

641.6432

115.8

4.06(5)

2.54(3)

3.74(5)

[7]

10–9 9–8

649.7015

175.3

3.489(94)

2.615(99)

3.298(99)

This work

3.71(7)

2.12(4)

3.38(6)

[7]

6.3(18)

2.7(6)

5.6(15)

[6]

14–13 13–12

2497.8090 2497.8123

Error value in parenthesis is 3s uncertainty of Dgfit. Error value in parenthesis is 3s uncertainty of Dgtotal. This line is used by the SMILES mission. d This line is used by the AURA MLS mission. b

Elower (cm  1)

F 0 2F 00

191.0

This work

A. Mizoguchi et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 113 (2012) 279–285

We discuss the advantages of the new production method as compared with the previous methods. In order to investigate a quantum number dependence of the pressure broadening coefficient, the result for the NKaKc ¼41 3– 31 2, J¼4.5–3.5 line at 265.77 GHz is also presented. Further, the effect of an unpaired electron on the pressure broadening coefficient is discussed. 2. Experimental procedure The measurements were carried out using a submillimeter-wave spectrometer that employs a backward wave oscillator (BWO; Istok, OB-80-1 and OB-32) as the radiation source. Fig. 1 shows a schematic diagram of the experimental setup. Details of the spectrometer have been described elsewhere [8]. The frequency of the BWO was stabilized with a phase-locked loop to a microwave synthesizer using a harmonic mixer. The submillimeterwave radiation passing through a 2 m long absorption cell was detected by an InSb detector cooled by liquid helium. The accuracy for measurement of a transition frequency is about 100 kHz. Commercially available N2 and O2 gases (purity 499.995%) were used as buffer gases. In this experiment, all measurements were carried out at room temperature (25–27 1C). The HO2 radical was produced by the mercury-photosensitized reaction of H2 (purity499.995%) and O2. Feeding of both precursors were regulated to a flow rate of 0.6 sccm by a mass-flow controller and slowly evacuated by a rotary pump. The pressure was monitored by two capacitor manometers set at the inlet and outlet of the cell. The flow speed was controlled so that the pressure difference between the two gauges was less than 1%, and the average of the two manometer values was taken as the internal pressure. The precursor gases were passed through a mercury reservoir with a magnetic stirrer before introduction to the cell. The partial pressure of Hg was expected to reach the saturated vapor pressure of 9  10  7 Torr at 26 1C. A 1.5 m long UV mercury lamp set in the cell excited the ground state Hg atoms into the 3P state. Then, HO2 was produced by the following reaction [9]: Hgð1 SÞ þ hnð253:7 nmÞ-Hgn ð3 PÞ

Hgn ð3 PÞ þ H2 ð1 S Þ-Hgð1 SÞ þ 2Hð2 SÞ Hð2 SÞ þO2 ð3 S Þ þ M-HO2 ð2 A0 Þ þ M

3. Results and analysis The target rotational transition at 625.66 GHz has a hyperfine doublet structure consisting of F ¼10–10 and 11–11 components. These lines were measured by the frequency modulation method at 625.6603 GHz and 625.6638 GHz, respectively. These frequencies agree to the spectroscopic catalog [10] within an experimental accuracy of 100 kHz. Two sets of line profiles with increases of pressure up to 1 Torr of N2 or O2 buffer gas are shown in Fig. 2. Since the pressure broadening parameter of N2 is larger than that of O2, the doublet feature disappears more easily in the case of N2. The same measurement but of the 649.70 GHz transition is shown in Fig. 3. As the splitting between two hyperfine components, F¼9–8 and F¼ 10–9, is only 36 kHz, the appearance is that of a single line within its Doppler broadening. The S/N ratio observed is about 10 times lower than that of the 625.66 GHz line because of less radiation power from the BWO. Hence, the measurement of the pressure broadening coefficient was limited to pressures less than 400 mTorr.

PC Mass Flow Controller

Function Generator

H2

2kHz, VFG

O2

AC Amp. Hg

Phase Locked BWO

Liquid He Cooled InSb Detector Hg Lamp

ð1Þ

Since the amount of HO2 is limited, mainly by such a low Hg vapor pressure, it is not easy to detect HO2 by the direct absorption method. Thus, Zeeman modulation technique was employed to obtain higher sensitivity. Only paramagnetic species with large g-factor are detected in Zeeman modulation method. Therefore, neither diamagnetic precursors nor reaction products can be detected in this way. Moreover, this molecular modulation method is free of any interference noise that depends on the frequency of the spectrometer. However, a wavy baseline structure was left in the observed spectrum at the highest sensitivity. As described in the next section, it was necessary to subtract the baseline by fitting a polynomial function. The magnetic field was generated by a solenoid coil around the absorption cell. A sinusoidal current of 2 kHz generated the magnetic field of at most 18 Gp–p.

Lock-in Amp.

Rotary Pump

281

Pressure Gauge DC Power Supply

R

Fig. 1. Schematic diagram of experimental setup.

buffer gas N2 / O2

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905 [mTorr]

974 [mTorr]

780 896 606

740

465 538 315 324 174 136

94 F = 11 - 11

39

63

F = 10 - 10 625.64

625.65

625.66

625.67

625.68

625.69

625.64

625.65

Frequency / GHz

625.66

625.67

625.68

625.69

Frequency / GHz

Fig. 2. Pressure dependence of the spectra of the NKaKc ¼101 9–100 10, J ¼ 10.5–10.5 transitions in the ground vibronic state X2A0 of HO2 for (a) the N2 buffer gas and (b) the O2 buffer gas. The spectral profile simulated by the Pickett convolution method is superimposed with a red line. The numbers at the lefthand side represent the buffer gas pressure. The vertical scale of each profile is normalized by its peak intensity.

Finally, the series of spectra at 265.77 GHz broadened by N2 is shown in Fig. 4. We chose this N ¼41 3–31 2, J¼ 4.5–3.5 transitions in order to ascertain the rotational quantum number dependence of the broadening parameter. This transition also splits into two hyperfine components of F¼4–3 and F¼5–4 at low pressures. The strong signal makes it possible to measure the broadening at pressures up to 2 Torr. As reported by Morino and Yamada, no significant differences in g were observed for the hyperfine components of the J¼1–0 rotational transition of HCl [11]. Assuming that the pressure broadening and shift are identical for each hyperfine component in a rotational transition, we employed a convolution method [12] to extract the linewidth broadened by the buffer gas. So far, this method has been successfully applied to spectral analyses for complicated line profiles with hyperfine splitting [13]. The pressure broadened spectrum observed is fitted to a simulated line profile that is a convolution of a reference spectrum and a Lorentzian. The so-called reference spectrum is that observed under the lowest pressure condition, and the collisional effects of the buffer gas are included in the Lorentzian. The line profile F(i) is described by FðiÞ ¼ A

N X k¼1

RðkÞ

4. Discussion

Dn2 Dn2 þ ½ðikÞf þ s2

þ a0 þa1 i þ a2 i2 þ   

where R(k) is the line profile of the reference spectrum, A is a free parameter for the amplitude, Dn is the linewidth of the Lorentzian (HWHM), f is the step size of the frequency scanning, s is the frequency shift due to pressure, and N is the number of sampling points. In addition, polynomial terms are employed for baseline correction. The linewidths thus determined for the 625.66 GHz, 649.70 GHz, and 265.77 GHz lines are plotted against the buffer gas pressures in Fig. 5. Error bars on data points indicate 3s uncertainty limits, where s is the standard deviation of the individual least-squares fitting to Eq. (2). The filled and open markers represent the linewidths for N2 and O2 buffer gases, respectively. These data were measured over several days, so the day-to-day fluctuation is shown by different marker shapes. The pressure broadening coefficients were determined by fitting these data to a linear function from the origin. All the coefficients determined are listed in Table 1. The value of g(N2) is almost same at the 625.66, 641.64, and 265.77 GHz lines but is smaller at the 649.70 GHz line. Further, there is a significant discrepancy between our g(N2) results and those of Yamada [7]. The situation is similar for g(O2).

ði ¼ 1,2,. . .,NÞ

ð2Þ

Firstly, we discuss the total experimental uncertainty, because the origin of the error is not confined to the spectral line fitting process mentioned above. According

A. Mizoguchi et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 113 (2012) 279–285

283

375 [mTorr]

204 [mTorr] 282

158 192

113

80

55

40

649.69

649.70

649.71

649.69

649.70

Frequency / GHz

649.71

Frequency / GHz

Fig. 3. Pressure dependence of the spectra of the NKaKc ¼ 102 9  92 8, J ¼9.5  8.5 transitions for (a) N2 buffer gas and (b) O2 buffer gas.

where Dgfit is the error in spectral fitting, DgT is the error due to temperature fluctuation, and Dgp is the error in measuring the pressure within the cell. In general, the temperature dependence of g is described by the following equation:  n T g ¼ g0 ð4Þ T0

HO2 (N = 413 - 312, J = 4.5 - 3.5) in N2

1754 [mTorr] 1350 909 678

where g0 is the pressure broadening coefficient determined at temperature T0 (¼ 299 K). Using a Taylor expansion of Eq. (4) and neglecting terms above second order, the error DgT arising from temperature can be estimated as

464 257

34

0

265.74

265.75

DgT ¼ F=4-3

265.76

F=5-4

265.77

265.78

265.79

265.80

Frequency / GHz Fig. 4. Pressure dependence of the spectra of the NKaKc ¼ 41 3–31 2, J ¼4.5–3.5 transitions for N2 buffer gas.

to a previous report [8], the total error Dgtotal is given by

Dgtotal ¼

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Dg2fit þ Dg2T þ Dg2p ,

ð3Þ

ng0 DT: T0

ð5Þ

Assuming that the interaction between HO2 and N2 or O2 is caused only by electric dipole–quadrupole interaction, the temperature dependence of the linewidth should be proportional to T  5/6; that is, n ¼ 5/6 [14]. This simple theoretical model was experimentally supported by Yamada et al. with BrO radical [13]. They measured temperature dependence of g with N2 and O2 and showed the discrepancy between the observed and the theoretical values is less than 10% and 15%, respectively. As the temperature fluctuation was less than 2 K in all experiments, DgT is estimated at not more than 0.0056g0 by Eq. (5). The uncertainty in the pressure measurement Dgp arises from the capacitance manometer itself, which has an error specification of 0.25%, and from the pressure difference in the flow cell. Considering a systematic error

differential linewidth (HWHM) / MHz

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A. Mizoguchi et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 113 (2012) 279–285

4 N2 3

2

differential linewidth (HWHM) / MHz

O2

1

0 0.0

differential linewidth (HWHM) / MHz

of 1% in the pressure measurement, the broadening coefficient varies by the same factor, i.e., Dgp ¼0.01g0. Therefore, using Eq. (3), the total uncertainties Dgtotal for N2 and O2 broadening of the 625.66 GHz line are estimated at 0.069 MHz/Torr and 0.058 MHz/Torr, respectively, which correspond to 1.7% and 2.2% of g0. Similarly, the total uncertainties Dgtotal for N2 and O2 broadening of the 649.70 GHz line are estimated at 0.098 MHz/Torr and 0.104 MHz/Torr, respectively, which correspond to 2.8% and 4.0% of g0. The total uncertainty Dgtotal for N2 broadening of the 265.77 GHz line is estimated at 0.084 MHz/Torr, which correspond to 2.1% of g0. The air broadening coefficient, g(air), can be estimated from g(N2) and g(O2) as

0.2

0.4 0.6 0.8 differential pressure / Torr

1.0

2 N2

1 O2

0 0.0

0.2 0.4 differential pressure / Torr

0.6

8

N2

6

4

2

0 0.0

0.4

0.6 1.2 differential pressure / Torr

1.6

Fig. 5. Plot of the pressure-broadened differential linewidth of HO2 in HWHM versus the differential pressure of the N2 or O2 buffer gas for the transitions: (a) NKaKc ¼ 101 9–100 10, J ¼10.5–10.5, (b) NKaKc ¼ 102 9–92 8, J ¼ 9.5–8.5, and (c) NKaKc ¼ 41 3–31 2, J ¼4.5–3.5. The filled and open markers show the linewidths for N2 and O2 buffer gases, respectively. These data were measured over several days, so the day-to-day fluctuation is shown by different marker shapes. The g coefficients determined by linear regression analysis are (a) g(N2)¼ 4.085 7 0.049 MHz/Torr, g(O2) ¼2.578 7 0.047 MHz/Torr, (b) g(N2) ¼ 3.489 7 0.094 MHz/Torr, g(O2) ¼ 2.615 70.099 MHz/Torr, and (c) g(N2) ¼ 4.049 7 0.065 MHz/Torr.

gðairÞ ¼ gðN2 Þ  0:79 þ gðO2 Þ  0:21

ð6Þ

where 0.79 and 0.21 are the fractions of N2 and O2 in air. Thus, using Eq. (6), the air broadening coefficient is 3.76970.067 for the 625.66 GHz line and 3.2987 0.099 MHz/Torr for the 649.70 GHz line. The errors reflect the 3s uncertainty in the least-squares fitting for the determination of g. This experimental accuracy satisfies the requirement of 3% for the SMILES mission. However, we still have to be careful in extrapolating it to stratospheric temperature since the temperature depending exponent n in Eq. (4) is not determined experimentally. Next, we discuss the production method for the HO2 radical. In a previous measurement, Chance et al. produced the HO2 radical by DC discharge in He, O2, Cl2, and CH3OH. They reported that several intermediate species with large dipole moments, such as H2CO and HCl, were produced as byproducts. In a measurement of the pressure broadening coefficient, undesired molecules with a large dipole moment will perturb the target molecule by the intermolecular electric dipole–dipole interaction, which is stronger and of longer range than the electric dipole–quadrupole interaction. Such an influence will prevent accurate determination of the pressure broadening coefficient, particularly for the lower pressures because the relative abundance of the undesired molecules in the buffer gas becomes larger. However, none of the precursors (H2, O2, and Hg) used in this experiment have a dipole moment, and the only intermediate species in reaction (1) is the hydrogen atom. Therefore, this would have been favorable to measurement of the pressure broadening coefficients with less influence from undesired species. As mentioned in the previous section, there are significant discrepancies in g(N2) and g(O2) for 649.701 GHz between our results and those of Yamada [7], who used the DC discharge of O2 and CH2 ¼CHCH2OH. Yamada et al. did not mention much about the reaction chemistry in the cell; however, they did indicate that the HO2 transition at 649.701 GHz was difficult to analyze because there was a strongly obstructive unknown line nearby. Our measurement was free from any such accidental overlapping by an unknown line and thus should be more reliable. The value of g(N2) for NKaKc ¼132 12–121 11 seems extremely large compared with others in Table 1. However, since that value has a large uncertainty of 29%, it has been neglected in this discussion. The collisional cross section between molecules is related to the mutual internal

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285

Table 2 Comparison of pressure broadening coefficient of HO2 with those of similar molecules. Frequency (GHz)

Elower (cm  1)

g(N2) (MHz/Torr)

g(O2) (MHz/Torr)

Reference

HO2 (2.09 D) NKaKc ¼ 101 9–100 10, J¼ 10.5–10.5 NKaKc ¼ 101 10–91 9, J¼ 9.5–8.5 NKaKc ¼ 102 9–92 8, J¼ 9.5–8.5

625.66 641.64 649.70

119.4 115.8 175.3

4.085(49) 4.06(5) 3.489(94)

2.578(47) 2.54(3) 2.615(99)

This work [7] This work

HCN (2.98 D) J¼ 9–8 J¼ 10–9

797.43 885.97

106.4 133.0

4.77(5) 4.46(5)

2.73(3) 2.91(3)

[15] [15]

H2O2 (1.57 D) JKaKc ¼71 6–80 8 JKaKc ¼201 19–192 17

223.10 625.04

61.6 362.0

4.87(36) 4.03(6)

2.85(36) 2.49(4)

[16] [8]

Error is 3s uncertainty.

rotational energy levels because energy transfer between the counterparts is resonantly enhanced. The energy level of the asymmetric rotor HO2 is rather complicated; therefore, accidental coincidence of the rotational energy separation with that of the counterpart may occur. This may be one reason for the irregular dependence of the g value on the rotational state. More generally, it is known that the pressure broadening coefficients become smaller with an increase in the rotational term value. This is because a faster rotational motion better averages out the effect of the electric dipole moment, which is the most significant element for intermolecular interaction. Another interesting effect on the collisional cross section is that of unpaired electrons in radicals. Table 2 compares the pressure broadening coefficients of HO2 with those of HCN [15] and H2O2 [8,16], which are almost the same in size. Despite the differences in permanent dipole moment and rotational term value, the pressure broadening coefficients are around 4 and 2.5 for the N2 and O2 buffer gases, respectively. No effect due to the magnetic interaction between doublet HO2 and triplet O2 is recognized here. This result suggests that the pressure broadening coefficient in the ground state is not affected by the existence of the unpaired electron. 5. Conclusion Using a mercury-photosensitized reaction, we determined two pressure broadening coefficients of the ground state HO2 radical that have been monitored by the SMILES mission. One is for the 625.66 GHz line and is newly observed. The other is for the 649.70 GHz line and its value has been corrected by 10%. These two data are proposed for use in the SMILES project. It would be very useful for validation of the altitude abundance derived from the SMILES spectral data if two independent lines were prepared for retrieval processing.

Acknowledgments Our work was supported in part by the SMILES project of NICT. A.M. thanks Dr. H. Habara for providing the program

to analyze the pressure broadening coefficients. We are grateful to Dr. M.M. Yamada and Prof. T. Amano for discussions about pressure broadening coefficients. We are also grateful to Dr. Y. Kasai for constant encouragement. References [1] Brasseur GP, Orlando JJ, Tryndall GS. Atmospheric chemistry and global change. Oxford University Press. [2] Zhang M, Akimoto H, Uno I. A three-dimensional simulation of HOx concentrations over East Asia during TRACE-P. Journal of Atmospheric Chemistry 2006;54:233–54. [3] Waters JW, Read WG, Froidevaux L, Jarnot RF, Cofield RE, Flower DA, et al. The UARS and EOS microwave limb sounder (MLS) experiment. Journal of Atmospheric Science 1999;56:194. [4] NASDA, JEM/SMILES Mission Plan Version 2.1, NASDA/CRL, November 2002. [5] Sato TO, et al. Unpublished results. [6] Chance K, De Natale P, Bellini M, Inguscio M, Di Lonardo G, Fusina L. Pressure broadening of the 2.4978-THz rotational lines of HO2 by N2 and O2. Journal of Molecular Spectroscopy 1994;163:67–70. [7] Yamada MM. PhD thesis. Ibaraki University, 2003. [8] Sato TO, Mizoguchi A, Mendrok J, Kanamori H, Kasai Y. Measurement of the pressure broadening coefficient of the 625 GHz transition of H2O2 in the sub-millimeter-wave region. Journal of Quantitative Spectroscopy and Radiative Transfer 2010;111: 821–5. [9] Shimokoshi K, Mori Y, Tanaka I. ESR study of the mercury-photosensitized H2 þO2 reaction. Bulletin of the Chemical Society of Japan 1967;40:254–60. [10] Pickett HM, Poynter RL, Cohen EA, Delitsky ML, Pearson JC, Muller HSP. Submillimeter, millimeter, and microwave spectral line catalog. Journal of Quantitative Spectroscopy and Radiative Transfer 1998;60:883–90. [11] Morino I, Yamada KMT. Absorption profile of HCl for the J¼ 1–0 rotational transition: foreign-gas effects measured for N2, O2 and Ar. Journal of Molecular Spectroscopy 2005;233:77–85. [12] Pickett HM. Determination of collisional linewidths and shifts by a convolution method. Applied Optics 1980;19:2745–9. [13] Yamada MM, Kobayashi M, Habara H, Amano T, Drouin BJ. Submillimeter-wave measurements of the pressure broadening of BrO. Journal of Quantitative Spectroscopy and Radiative Transfer 2003;82:391–9. [14] Townes CH, Schawlow AL. Microwave Spectroscopy. Dover. [15] Yang C, Buldyreva J, Gordon IE, Rohart F, Cuisset A, Mouret G, et al. Oxygen, nitrogen and air broadening of HCN spectral lines at terahertz frequencies. Journal of Quantitative Spectroscopy and Radiative Transfer 2008;109:2857–68. [16] Goyette TM, Ebenstein WL, Shostak SL, DeLucia FC, Helminger P. Pressure broadening of NO2, CF2Cl2, HDO and HOOH by O2 and N2 in the millimeter wave region. Journal of Quantitative Spectroscopy and Radiative Transfer 1988;40:129–34.