Journal of Molecular Spectroscopy 221 (2003) 139–141

www.elsevier.com/locate/jms

Letter to the editor

The bend vibration excitation of SOþ2 ( ~DD) in the range of 291–312 nm

SO2 and the ions derived from it could play im-portant roles in atmospheric chemistry, environmental

pollution, and industry, such as in the overall chemis-

try of the dry etching process [1,2]. The photoelectron

spectroscopy with molecular beam [3,4] and the pho-

toionization [5,6] of SO2 molecule have been used to

provide the vibration structure for the electronic states

of SOþ2 including the overlapped ~EE; ~DD; ~CC states. The

common ground of above methods is to excite theneutral molecule SO2 by using high energy photons.

The only available photodissociation spectrum of SOþ2ion by the direct photo-excitation of SOþ2 itself, as

known by us, is from Thomas et al. [7]. In their study,

the SOþ2 ions prepared by electron impact were selected

by a triple quadrupole system and the photodissocia-

tion spectrum of SOþ2 ion was obtained by the coaxial

irradiation of tunable dye laser. However, the signal–noise ratio and resolution for their photodissociation

spectra of SOþ2 ion seem to be poor. Similar to our

previous study on the photofragment excitation

(PHOFEX) spectrum of CSþ2 [8], in this work the two

color method is used to study the PHOFEX spectrum

of SOþ2 . The SOþ2 ions were prepared by the [3 + 1]

REMPI of neutral SO2 molecules at 380.85 nm, the

photofragment SOþ excitation spectrum of SOþ2 ionwas obtained by scanning the second tunable pulsed

laser in the range of 291–312 nm. From the PHOFEX

spectrum of SOþ2 , the new bend vibrational structure of

SOþ2 ( ~DD) was found and was analyzed to yield the

harmonic bend vibrational frequency m2 and the an-

harmonicity constant X22.

The experimental setup has been reported previously

[8,9]. Briefly, it consists of (i) a pulsed molecular beamsource to generate the jet-cooled SO2 molecules, (ii)

two dye laser systems pumped by two YAG lasers with

the pulse width of �5 ns, respectively, and (iii) a home-

made time-of-flight (TOF) mass spectrometer. One dye

laser FL3002 (Lambda Physics), pumped by the THG

(354.7 nm) output of a Nd:YAG laser (LABest Op-

tronics), was used for photoionization. The second dye

laser HD-500 with frequency doubling HD-1000 (Lu-monics), pumped by the SHG (532 nm) output of a

Nd:YAG laser GCR-170 (Spectra-Physics), was used

for photodissociation. The output of photoionization

dye laser (380.85 nm, �1.5mJ/pulse) was focused per-

0022-2852/$ - see front matter � 2003 Elsevier Science (USA). All rights res

doi:10.1016/S0022-2852(03)00167-X

pendicularly on the molecular beam of SO2 by a quartzlens with f ¼ 120mm and was used to prepare SOþ2molecular ions via [3 + 1] REMPI of SO2 [10]. The

photodissociation dye laser, with the output of

�0.1mJ/pulse in the range of 291–312 nm, was weakly

focused by another quartz lens (f ¼ 500mm) and was

employed to dissociate SOþ2 ions via one-photon exci-

tation.

The [3 + 1] REMPI spectrum of SO2 molecule in therange of 365–405 nm has been studied in detail [9,10].

The resonance band located at k ¼ 380:85 nm was used

in this study owing to dominating parent ion SOþ2 and

very few fragment ions SOþ and Sþ produced at this

wavelength. The amounts of SOþ and Sþ ions were less

than 1/15 of SOþ2 in our experiment. The [3 + 1] REMPI

of SO2 at k1 ¼ 380:85 nm can be expressed as

SO2ð ~XX 1A1Þ !3hv1

SO2ð4pa1 ~GGð000ÞÞ!hv1

SOþ2 ð ~XX 2A1Þ þ e;

where 4pa1 ~GG represents for the ~GG state with a Rydberg

orbital of 4pa1. The ~GG Rydberg state with the A1 sym-

metry converges to the SOþ2 ground state ð ~XX 2A1Þ [10].Though the 13.00 eV energy of four photon with

k1 ¼ 380:85 nm can excite SO2 ( ~XX 1A1) to a position just

above SOþ2 ( ~AA2A2) at 12.99 eV, the population of SOþ2( ~AA2A2) can be omitted, at least, owing to the unfavorable

transition of SOþ2 ( ~AA2A2) SO2 ( ~GGA1).By carefully controlling the intensity of the dissocia-

tion laser, no ion signal could be observed only with this

laser, but the remarkably strong SOþ signal appeared in

TOF mass spectrum with the addition of the ionization

laser of k ¼ 380:85 nm by which alone the dominating

parent ion SOþ2 and very few fragment ions SOþ were

produced. It means that most of the SOþ ions should be

the photodissociation products of SOþ2 . Fig. 1 shows thePHOFEX spectrum obtained in the wavelength range of

291–312 nm with the ionization laser fixed at 380.85 nm.

In the entire scan range the signal of SOþ ion is domi-

nated compared to the very weak Sþ ion signal. The

obvious resonance bands are shown in the PHOFEX

spectrum by measuring SOþ. To avoid the disturbance

from autoionization of neutral SO2, there were about

60 ns delay temporally and the slight separation in thedirection of ion flight between the dissociation laser

and ionization laser in the laser-molecular interaction

region.

erved.

Fig. 1. The PHOFEX spectrum of SOþ2 obtained by monitoring SOþ

ions in the wavelength range of 291–312 nm. The new bend vibration

series is marked by ‘‘*’’. The assignment of spectra refers to the

photoelectron spectra of [3,4]. No correction is made in the laser

efficiency. The laser efficiency curve was given at the top of spectra in

figure.

140 Letter to the editor / Journal of Molecular Spectroscopy 221 (2003) 139–141

According to the spectroscopic data obtained from

previous studies on the photoelectron spectroscopy of

SO2 [3,4], the one photon energy in the wavelengthrange of 291–312 nm can excite the allowed transitions

of SOþ2 ( ~EE; ~DD; ~CC) SOþ2 ( ~XX 2A1) [11], where ~EE, ~DD, ~CC pos-

sess one kind of 2B2;2B1, and 2A1 symmetry, respec-

tively. The possible assignments for the PHOFEX

spectrum were tried, and it is found that if only to assign

the PHOFEX spectrum to the transitions from the vi-

bration level (000) of SOþ2 ( ~XX 2A1) the complete vibration

series could be obtained for the PHOFEX spectrum, asshown in Fig. 1 and Table 1. This means that in the

[3 + 1] REMPI of SO2 the excited SO2 (4pa1 ~GGð000Þ)converges to SOþ2 ( ~XX 2A1ð000Þ) finally.

The assignments of ~DDðv1 ¼ 0� 2; v2 ¼ 0; v3 ¼ 0Þ ~XX 2A1ð000Þ and ~EEðv1 ¼ 0; 1; v2 ¼ 0� 3; v3 ¼ 0Þ ~XX 2A1

ð000Þ of SOþ2 were deduced from the data of photo-

electron spectra [4] and were shown in Fig. 1. To assign

other resonance bands in PHOFEX spectrum we firsttried to assign them to SOþ2 ( ~CC) SOþ2 ( ~XX 2A1) transi-

tion. However, it failed to find the series of SOþ2( ~CC) SOþ2 ( ~XX 2A1ð000Þ) transition in the PHOFEX

spectrum of 291–312 nm, although the allowed transi-

tions of SOþ2 ( ~CCð410; 500; 510; 600; 610; 700Þ) SOþ2

Table 1

Band positions and assignments for the fragment SOþ excitation

spectrum of SOþ2

Transition of SOþ2 ~XX 2A1ð000Þ Wavenumbers

(cm�1)

Spacing

(cm�1)~DDð0v20Þ

000 32173.0 0

010 32404.4 231.4

020 32641.3 236.9

030 32873.1 231.8

040 33099.4 226.3

050 33330.0 230.6

060 33550.3 220.3

070 33767.8 217.5

080 33977.8 210.0

090 34190.4 212.6

( ~XX 2A1ð000Þ) predicted by the newly PES spectrum [3,4]should locate in this wavelength range. To inspect the

PHOFEX spectrum at a wavelength longer than 311 nm

(at energy below the SOþ2 (D) state) we measured the

PHOFEX spectrum in the range of 312–332 nm where

much smaller SOþ signal was produced. From the

PHOFEX spectrum in the range of 312–332 nm no ob-

vious vibrational progressions of SOþ2 ( ~CC) could be

identified. Moreover, it was found that the weak PHO-FEX spectrum in the range of 312–332 nm can be ob-

tained only by increasing the intensity of dissociation

laser to about 3 times of that used in the range of 291–

312 nm. This fact means that it is hard to find the clear

resonance bands at wavelengths longer than 311 nm (at

energy below the SOþ2 ( ~DD) state) and the possibility of

SOþ2 ( ~CC) SOþ2 ( ~XX 2A1ð000Þ) transition is very small.

Finally, if the absolute wavelength position predicted byPES was disregarded (although it seems not reasonable),

we may assign the peaks at 308.60, 304.20, and

300.03 nm to (310), (400), and (410) levels of SOþ2 ( ~CC),respectively, by referring to [7]. In this way, the deduced

m2 value of SOþ2 ( ~CC) seems reasonable (�460 cm�1), butthe deduced m1 value of �920 cm�1 of SOþ2 ( ~CC) at v1 ¼ 3

(it should be about v1 ¼ 4 according to newly PES data

in [3,4]) seems too large in comparison with the values of767 and �780 cm�1 predicted by [7] and by [3,4], re-

spectively. In fact, the m1 value of �920 cm�1 seems to be

more like that of SOþ2 ( ~DD) as shown in [3,4]. Therefore,

we give it up to assign SOþ2 ( ~CC) SOþ2 ( ~XX 2A1ð000Þ)transition in PHOFEX spectrum.

It is very interesting to note that the three resonance

bands with the adjacent space of �230 cm�1, located at

304.20, 306.36, and 308.60 nm, respectively, are so reg-ular that it makes us think that they should be related to

each other by a spectral series, however, they cannot be

assigned reasonably to C–X transitions by using the

available spectral data of SOþ2 [3,4] as mentioned above.

Owing to the energy positions of these bands which are

between SOþ2 ( ~DDð000Þ) and SOþ2 ( ~EEð000Þ), it is reason-

able to assign these bands to the transitions of~DDð0v20Þ ~XX 2A1ð000Þ for SOþ2 , where v2 is the bendvibration quantum numbers. The important support for

this assignment is the large change of molecular bond

angle from 136.5� of SOþ2 ( ~XX 2A1) to 119.5� of SOþ2 ( ~DD),which is favorable to excite the transition to the bent

vibration levels of SOþ2 ( ~DD) from SOþ2 ( ~XX 2A1). The bend

vibration excitation of SOþ2 ( ~DD) was not given in pho-

toelectron spectrum [3,4], maybe owing to the nearly

same molecular bond angle of 119.5� for SOþ2 ( ~DD) andSO2 ( ~XX 1A1) [12], which is unfavorable to excite the bent

vibration mode of SOþ2 ( ~DD) from SO2 ( ~XX 1A1). Following

this assignment, the bend vibration levels from v2 ¼ 0

to v2 ¼ 9 of SOþ2~DDð0v20Þ were obtained, as shown in

Table 1. New harmonic bend vibrational frequency

m2 ¼ 241:8� 0:9 cm�1 and the anharmonicity constants

X22 ¼ �1:7� 0:1 cm�1 for SOþ2 ( ~DD) were deduced by

Letter to the editor / Journal of Molecular Spectroscopy 221 (2003) 139–141 141

using least-squares fitting. However, the lower m2 of�240 cm�1 for ~DD state given by us is out of the range of

454� 100 cm�1 predicted by Thomas et al. [7]. Further

evidence both in theory and in experiment (such as laser-

induced fluorescence excitation spectrum) is needed to

check the validity of this temporary assignment. If the m2value of �240 cm�1 for ~DD state is correct, it means that

the potential curve of ~DD state is not similar to other

states along the bend vibrational coordinate. We alsotried to assign in the PHOFEX spectrum the sum fre-

quency excitation of m1 and m2 of ~DD state, such as ~DD(110), ~DD (210), etc. Unfortunately, the diffusion of

spectrum prevented us from getting precise assignment

to them. So no sum frequency was shown in Fig. 1 and

in Table 1. It is worth to notice that the rotational

structure of resonance bands in PHOFEX spectrum

cannot be resolved even with a spectral resolution of�0.1 cm�1 from dissociation laser. Whether the shape of

resonance band comes from the distribution of rota-

tional transition or not, and whether the broader band,

such as at 306.36 nm, comes from multiple contributions

or not, these problems are still to be answered in the

further study.

In conclusion, this work has provided the much bet-

ter photofragment SOþ excitation spectrum of SOþ2comparing to the previous study [7], the new bend vi-

bration series of SOþ2 ( ~DD) has been assigned and the

harmonic bend vibrational frequency m2 ¼ 241:8�10:9 cm�1 and the anharmonicity constants X22 ¼ �1:7�0:1 cm�1 for SOþ2 ( ~DD) have been obtained.

In this letter we do not deal with the symmetry of ~DDstate and the predissociation mechanism, which will be

discussed elsewhere.

Acknowledgments

Support from the National Natural Science Foun-

dation of China (No. 20173053) and the National Key

Basic Research Special Foundation is gratefully ac-knowledged.

Zhong Wang

Limin Zhang

Jiang Li

Feng Wang

Shuqin Yu

Laboratory of Bond Selective Chemistry

Department of Chemical Physics

University of Science and Technology of China

Hefei, Anhui 230026

People�s Republic of China

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