A high-resolution study of the vacuum ultraviolet spectrum of CS: The B1Σ+-X1Σ+ system

JOURNAL OF MOLECULAR

SPECTROSCOPY

124,420-429 (1987)

A High-Resolution Study of the Vacuum Ultraviolet Spectrum of CS: The B’ 2+-X12+ System G. STARK, K. YOSHINO, AND P. L. SMITH Harvard-SmithsonianCenterfor Astrophysics,60 Garden Street, Cambridge, Massachusetts02138 A high-resolution, vacuum ultraviolet absorption study of CS was performed. CS absorption features in the 1280 to 1550-A region were photographed and measured. Four bands (O-O, l-0, 2-0, l-1) of the ‘*C3*SB’Z+-X’Z+ system were observed, plus the 1-O bandheads of ‘3C32Sand ‘%?S. A rotational analysis of the ‘*C3*S1-Oband, the only band with resolved rotational structure, was completed. The other bands in the B-X system were observed to tx diffuse, presumably because of predissociation of the B’Z+ state. The derived equilibrium molecular constants of the B’Z+ state were found to be close in value to those of the *X+ ground state of the CS+ ion, indicating that the B’Z+ state is a Rydberg state of CS. All absorption features shortward of the B-Xsystem were found to be diffuse. Descriptions and wavenumbers of these features, which are tentatively identified as CS transitions, are presented. 0 1987 Academic press, IIK. 1.

INTRODUCTION

Carbon monosulfide is found in a variety of astrophysical settings, including dense interstellar clouds (I, 2), star-forming regions (3), carbon-rich stars (4, 5), and comets (6, 7). CS has also been searched for, but not detected, in diffise interstellar clouds (8-10). CS emission and absorption features are used to determine number densities, isotopic abundances, and excitation conditions in these sources. The CS spectrum has been studied extensively in the near ultraviolet. Emission bands associated with the A’ ‘Z+-X’Z+ transition (1700-2400 A) have been observed in a microwave discharge (1 I). A thorough investigation of the emission and absorption bands of the strong A’II-X’Z+ system (2500-3 100 A) has been published by Bergeman and Cossart (12). These authors also identified and analyzed a number of tripletsinglet transitions in this region. Relatively few spectroscopic studies of CS have been carried out in the vacuum ultraviolet. Low-resolution absorption spectra, following the flash photolysis of CS2, have been reported by Donovan et al. (13). These authors identified a number of new bands in the 1200- to 1600-A region; however, rotational analyses were not possible. More recently, a resonance-enhanced, multiphoton ionization scheme has been used to detect the presence of a high lying (-63 000 cm-‘) triplet state (32’) in CS (14). Rotational assignments of four bands of the 32+-a311 transition were reported. In this paper, we describe a high-resolution, vacuum ultraviolet CS absorption study. We report data on four vibrational bands in the ‘2C32SBIB+-X’Z+ system. A rotational analysis of the 1-O band, the only band with observable rotational structure, is presented. We also report the observation of the 1-O bandheads of the isotopic species ’3C32Sand 12C34S.In addition, a number of absorption features in the 1280- to 14 1OA region, also identified with the CS molecule, are described. 0022-2852187 $3.00 Copyright 0 1987 by Academic Pros,

Inc. AU rights of reproduction in any form reserved

420

VACUUM

ULTRAVIOLET

2. EXPERIMENTAL

SPECTRUM

421

OF CS

DETAILS AND RESULTS

The CS molecule was produced in a side arm attached to a 25cm-long absorption cell, in a silent, ac discharge through argon and CS2. CS2vapor was admitted through a needle valve in the side tubing containing the discharge electrodes. A reservoir of CS2 liquid (Fischer Scientific reagent grade CS2) at 0°C provided a constant vapor pressure of N 100 Torr behind the valve. The liquid was degassed by repeated freeze/ pump/thaw cycles and used without further purification. The estimated CS2 vapor pressure in the discharge tube varied between 0.1 and 1 Tot-r. An argon flow at 5 Torr was used to inhibit the deposition of discharge products on the magnesium fluoride windows of the absorption cell. A mechanical pump, protected with a liquid nitrogen trap, completed the flow system. For absorption measurements, a condensed ac discharge in 400 Torr of krypton (25) was used to produce a continuum extending from - 1250 to - 1600 A. Spectra were photographed on a 6.65-m McPherson model 265 vacuum spectrograph in the first order of a 2400 grooves/mm grating coated with magnesium fluoride. The reciprocal dispersion was approximately 0.61 &mm and an entrance slit width of 20 pm was employed. Typical exposure times ranged from lo-30 min on Kodak SWR plates. Line positions were measured on a Grant photoelectric comparator with an uncertainty of 1 pm for unblended lines. Rotational lines of the A’II-XrZ+ system of CO, a contaminant in the krypton discharge, were used to calibrate the spectra. The CO rotational line wavelengths were obtained from the known term values of the A and X states of CO, the uncertainties of which are less than 0.05 cm-’ (26). Because CSp was not completely dissociated in the discharge, spectra were obtained with the CS, discharge both on and off, allowing for discrimination between CS and CS2 absorption features (17). Spectra were recorded from 1280 to 1560 A. Absorption features corresponding to the O-O, l-0, 2-0, and l-l bands of the B-X system, plus features at shorter wavelengths, were observed. A portion of the photographed CS absorption spectrum is shown in Fig. 1. All attempts to photograph the CS B-X system in emission failed. In this phase of the study, the CS2 discharge was placed directly in front of the spectrograph entrance slit. Spectra were recorded successively in the B-X region (1500- 1550 A) and in the A-X region (2500-2600 A). Strong A-X emission was observed in the Au = 0, 1, and 2 sequences. No emission in the wavelength region 1500- 1550 A was seen.

o;or

I;,“,

CO?)

62

lb3

Ib4”nl

FIG. 1. A portion of the VUV absorption spectrum of CS. In the lower exposure, the CS, discharge is off and CS2 absorption features are prominent. In the upper exposure, the discharge is on and the CS B(O)X(O), B( 1)--X(O), and B( 1)-X( 1) bands appear.

422

STARK, YOSHINO, AND SMITH 3. ANALYSIS AND DISCUSSION

3. I. Rotational Analysis of the B(I)-X(0) Band Of the four bands observed in the B-X system, only the 1-O band showed a rotationally resolved structure. This band is shown in Fig. 2. A rotational analysis of the band was straightforward. Thirty-six R-branch lines and six P-branch lines were identified (see Table I). Term values for 36 rotational levels in the upper state were calculated from the measured line wavenumbers and the known constants of the X’Z’+ ground state. The ground state of CS has been studied both in the microwave and infrared regions. The recent FTS data of Winkel et al. (18) were used to calculate ground state term values. The uncertainties in the ground state term values are far less than our measurement uncertainties, and therefore do not limit the accuracy of our results. The upper state term values were fit in a least-squares procedure to the formula E(J’) = T+ B’J’(J’+ I)-D’J’*(J’+

l)*.

(1)

The results are displayed in Table II. The rms deviation of the measured line wavenumbers from the fit wavenumbers is 0.077 cm-‘. No anomalies in individual line positions or intensities are seen. Although an accurate, quantitative measurement of linewidths from the photographic spectra was not possible, measurements of unsaturated lines indicated linewidths on the order of 1 cm-’ (compared to an instrumental linewidth of 0.5 -t 0. I cm-‘, and an estimated Doppler width of0.07 cm-‘). The lineshapes are clearly broadened from predissociation of the upper state. Combination differences for six pairs of lines sharing upper state rotational levels were compared to known ground state term value differences. A systematic shift of -0.2 cm-’ was found. This discrepancy is caused by a shift in the apparent P-branch line centers, due to the superposition of these lines on a strong, sharply sloping background (this background is due to wing contributions from P-branch lines near the bandhead). The generation of a synthetic spectrum of Lore&-broadened lines forming a bandhead verified that apparent line shifts of 0.1 to 0.2 cm-’ should occur.

FIG. 2. The CS B( 1)-X(O) band. To the right of the bandhead are marked the 1-O bandheads of “P’S and ‘%?S.

TABLE I Observed Lines of CS B( 1)-X(O) Band and Upper State Term Values (All Values in cm-‘) R(J)

0

66226.51 66228.30 66230.17 66232.01 66233.99 66236.07 66238.20 66240.42 66242.63 66244.88 66247.31 66249.82 66252.35 66254.94 66257.64 66260.28 66263.14 66266.08 66268.97 66271.99 66275.06 66278.24 66281.47 66284.71 66288.02 66291.50 66295.00 66298.58 66302.19 66305.91 66309.60 66313.37 66317.39 66321.24 66325.34 66329.43

1

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

P (J)

(0-c)

66221.34 66219.90 66218.43 66217.07 66215.87 66214.76

-0.20 -0.15 -0.19 -0.19 -0.10 -0.12

(0-c)

J

0.07 0.09 0.11 0.04 0.03 0.06 0.07 0.10 0.06 -0.03 0.00 0.04 0.04 0.02 0.05 -0.05 0.00 0.05 -0.01 -0.01 -0.02 0.01 0.02 -0.03 -0.07 -0.02 0.00 0.02 0.01 0.04 -0.03 -0.07 0.05 -0.05 0.02 0.04

Term

Value

66867.47 66870.89 66876.03 66882.77 66891.28 66901.54 66913.47 66927.13 66942.41 66959.36 66978.13 66998.60 67020.74 67044.56 67070.12 67097.26 67126.25 67156.94 67189.21 67223.24 67258.95 67296.40 67335.53 67376.29 67418.75 67462.99 67508.89 67556.49 67605.74 67656.72 67709.29 67763.56 67819.70 67877.28 67936.73 67997.79

TABLE II Vibrational Band 12@1S

Bands of the CS B’Z+-X’Z’ Origin

o-o

Head

64869(4) 66224.7(l) 67560(4)

1-o 2-o 1-l

System (All Values in cm-‘) B'

64852(4) 66204.6(j) 67537(4) 64934.(2)

IrpS

1-o

66167.4(10)

17C31S

1-o

66194.5(10)

(quoted

(@ See

text

errors

are

for discussion

in units

of the

of Bo and

D'

0.858@) 0.8520(3) 0 .846(*)

last

BI values.

1.56(10)x10-6

significant

figure)

STARK,

424 3.2. The O-O, 2-0, and l-l

YOSHINO,

AND

SMITH

Bands of the B-X System

Absorption features at 154 1 and 1480 A are identified as the O-O and 2-O bands of the B-X system, respectively. Both bands, though too diffuse to show rotational structure, are clearly violet-degraded, Z-2 transitions (the O-O band is shown in Fig, 1). Densitometer traces of the band profiles (see Fig. 3) were used to determine the band origins, which were identified with the central minima of the profiles. This procedure is only approximate (see Section 3.4). The values of the O-O and 2-O band origins are listed in Table II. The vibrational constants w, and 0,x,, as well as T,, the electronic energy of the B state, have been calculated from the O-O, l-0, and 2-O band origins. These values, plus calculated uncertainties, are presented in Table III. A weak feature at 1540.03 A was identified as the 1-l bandhead (see Figs. 1 and 3). This assignment is consistent, to within 0.04 A, with the 1-O band position and the known spacing of the vibrational levels of the X’I;+ ground state. No other absorption features originating from the V”= 1 level were seen (the 0- 1 and 2- 1 bandheads are expected to appear weaker than the l-l bandhead, due to a broadening of these features from predissociation of the upper states). 3.3. The B(I)-X(0)

Bands of ‘3C32S and ‘2C34S

The assignment of the 15 10 A band as the 1-O band of the B-X system is supported by the identification of two weak features to the long wavelength side of the bandhead as the 1-O bandheads of isotopic 13C3*Sand ‘2C34S. These features are shown in Fig. 2. The energy of a vibrational level of an isotopic molecule is given by G(u)=pw e(u+‘)--*w 2

e

x e (v+ ‘)* 2 7

(3)

where w, and mexe are the vibrational constants of the “ordinary” molecule, and P = (P/P Y,

L

I

64650

1

(4)

I

64950

67550

1

67650

WAVENUMBER FIG. 3. Densitometer traces of the B(O)-X(0) and &2)-X(O) with arrow) is the B( 1)-X( 1) bandhead.

bands. The feature at 64 934 cm-’ (marked

VACUUM

ULTRAVIOLET

SPECTRUM

425

OF CS

TABLE III Equilibrium Constants of the CS B’Z+ State (All Values in cm-‘) 64824(15) k) 1376.8(89) 10.3(28) 0.8617@)

T. % w*x. B. (quoted

errors

are

in units

of the

last

significant

(a) Electronic energy measured relative to the bottom potential curve. W See text for discussion of B. value.

figure)

of the

X lC+

where p and pi are the reduced masses of the “ordinary” and isotopic molecules, respectively. The rotational constants of isotopic and “ordinary” species are related by B’= p2B. (5) These relations predict 1-O bandheads for the ‘zC34Sand 13C3’Smolecules at 66 193.8 and 66 166.5 cm-‘, respectively. The measured positions are 66 194.5 and 66 167.4 cm-‘. The discrepancies between measured and calculated values (less than 1 cm-‘) are well within the estimated measurement errors. In addition, the strengths of the isotopic features are consistent with the naturally occurring abundance ratios: ‘2c32s:12c34s:13c32s = 1:0.044:0.011. 3.4. The Rydberg Nature of the B’F

State

The molecular constants of the B’Z+ state, derived from this study, are presented in Table III. The equilibrium rotational constant (B,) for this state could not be directly derived from the data because of the diffuseness of the O-O and 2-O bands. The value B, = 0.86 17 cm-’ was assigned after a comparison of the CS B ’ Z+ vibrational constants with the vibrational constants of the CS 2Z+ ground state, for which w, and o,x, are 1376.6 and 7.8 cm-‘, respectively (19). These values are within the experimental uncertainties of the B state w, and w,x, values, and indicate the Rydberg nature of the CS B’Z;+ state The rotational constants for the X22+ state are B, = 0.8673 cm-’ and (Y,= 0.0065 cm-‘. Assuming the same value for a, for the CS B state, consistent with its Rydberg nature, and using the well-determined value of B, (0.8520 + 0.0003 cm-‘), the rotational constants B,, Bo, and B2 can be derived (see Tables II and III). The derived values of B, is within 0.0056 cm-’ of the CS+ X2x+ ground state value. This result is consistent with the above assumptions. The derived values for B0 and B2 are consistent with the intensity profiles of the O0 and 2-O bands. If the energy difference (E) between the origin and head of a molecular band is known, B’ can be extracted (neglecting higher rotational constants) from the formula B’ = 2E - B” - 2(E2 _ 2B”E)‘/2. (2)

426

STARK, YOSHINO, AND SMITH

The O-O and 2-O bandheads and band origins were tentatively located at the points of maximum and minimum intensity in the band profiles. The Lorentz-broadening of the lines (due to predissociation of the upper state) and the line intensity distribution (due to the temperature-dependent population distribution of the ground state rotational levels) both affect the positions of the intensity maxima and minima. Because the O-O and 2-O bands appear completely diffuse, only a minimum average linewidth can be estimated. Computer-generated synthetic spectra of the O-O and 20 bands, with variable linewidths, indicate that all rotational structure is washed out for linewidths greater than 5 cm -‘. The line intensity distribution can be ascertained from the well-resolved R branch of the 1-O band, which indicates a rotational temperature of approximately 300 K for the D”= 0 level of the X’Z+ ground state. Synthetic spectra, taking into account both Lorentz-broadening of individual lines and a temperature-dependent line intensity distribution, indicate that the true bandhead positions and band origin positions are shifted by as much as 3 cm-’ from the points of maximum and minimum intensity. Synthetic spectra, using the values of the rotational constants & and B2 listed above, reproduced the important features of the measured intensity profiles of the O-O and 2-O bands (see Fig. 4). In particular, the synthetic spectra reproduced the measured separations between intensity maxima and minima. These spectra also generated corrected values for the O-O and 2-O bandheads and band origins. Both the measured and corrected values are listed in Table IV. The corrected values for the O-O and 20 band origins were used in the calculation of the B’Z+ equilibrium vibrational constants. 3.5. Comments on the Reported Observation of B State Emission The three observed vibrational levels of the B’Z+ state are clearly broadened by predissociation, especially the 2)= 0 and v = 2 levels. This explains our inability, and the inability of others (20), to observe the B-X system in emission. The reported observation of CS B-A bands in emission in a flowing afterglow by Yencha and Wu

I

I

I

I

I TRUE MEAS. TRUE MEAS.

67500

BAND BAND BAND BAND

87800

67550

HEAD = 67537.0 HEAD = 67540.7 ORIGIN = 67560.1 ORIGIN = 67561.0

..

87650

WAVENUMBER FIG. 4. A synthetic spectrum of the @2)-X(O) band. The rotational temperature is 300 K and the rotational Iinewidth is 6 cm-‘. The shifts in the apparent bandhead and band origin positions are indicated.

VACUUM

ULTRAVIOLET

427

SPECTRUM OF CS

TABLE IV Bandhead and Band Origin Positions of Diffuse B-X Bands (All Values in cm-‘) B-X

Band

Measured Head

o-o

64853.2 67540.7

2-o

Measured Origin

Corrected Head 64851.7 67537.0

64870.4 67561.0

Corrected Origin 64868.6 67560.1

(21) has been called into question by Cossart (20), who points out the discrepancies between Yencha and Wu’s AG A’II values and the known AG values of that state. The strong evidence now available of predissociation in the B ’ Z+ state indicates that only very weak, if any, emission features should be observable originating from the B state. While the spectroscopy of highly excited CS states is not complete enough to present a detailed interpretation of predissociation in the B state, our spectra clearly indicate that the B state is perturbed by more than a single electronic state. 3.6. Features Between 1280 and 1410 A’ Additional observed absorption features, in the wavelength range 1280 to 1410 A, are listed in Table V and shown in Fig. 5. Most of these features, which, with the exception of the last entry, are all diffuse, were first observed at low resolution by Donovan et al. (13). The first CS feature shortward of the B-X bands is a strong, diffuse band centered at 1400.8 A. The band consists of two symmetrical intensity maxima separated by an TABLE V CS Absorption Features between 1280 and 14 10 A

A (A)

0

(cm-‘)

1.

1400.8

71388.

2.

1399.8

71439.

1

1

IS? (strong)

(O-O)

E-X

(O-O)

(weak)

1396.8

71592.

3.

1392.0

71839.

Red

degraded

head

4.

1388.8

72005.

Red

degraded

head

5.

1361.9

73427.

ITI (strong)

6.

1360.6

73497.

(weak)

7.

1337.1

74789.

8.

c-x

Red

degraded

77507. (strong)

head

428

STARK, YOSHINO, AND SMITH (4)

(6k5)

136nm

(3)

136

IiOnm

(2)

(1)

140 nm

Ii2

Ii4nm

FIG. 5. CS absorption features in the wavelength range 128-141 nm. The numbering above the features corresponds to the descriptions in Table V. In the lower exposures, the CS2 discharge is off.

intensity minimum and is identified as a ‘Z+-‘LV transition. No bandhead is apparent. A group of weak features is seen just short of 1400 A (1396.8-1399.8 A). At the large number densities needed to see these features, they are partially obscured by both the CS ’ 2+-‘2+ band and CS2 bands. No characteristics leading to a definitive identification are found. The heads of two diffuse, red-degraded bands are observed at 1392.0 and 1388.8 A. The latter feature is significantly stronger than the former, yet weaker than the ‘I?+-‘Z+ transition at 1400.8 A. A strong, diffuse ‘II-‘.E+ transition is observed at 1361.9 A. No bandhead is seen. The intensity profiles of the P and R branches may indicate the presence of a perturbation. A very weak feature is observed at 1360 A. A relatively weak bandhead of a red-degraded band is observed at 1337.1 A. This feature closely resembles the heads seen at 1392.0 and 1388.8 A. A strong set of absorption features is observed between 1290.2 and 1288.5 A. Approximately 10 features are seen in this region-they seem to be converging to a short wavelength limit, although no sequence has yet been established. Resolved rotational lines are observed in some of the features. Donovan et al. tentatively assigned the short wavelength transitions observed in their flash photolysis experiment. Our high-resolution plates verify most of their assignments, with a few exceptions. Donovan et al. identified the 1388.8 and 136 1.9 A features as the (O-O) and (1-O) bands of the E’II-‘Z+ system. The 1388.8 A feature does not at ah resemble the 136 1.9 A feature, with its strong ‘II-‘Z+ characteristics. We tentatively identify the 1361.9 A feature as the (O-O) band of the E’II4’2’ transition. The set of features at 1289 A is labeled as a single ‘II-‘2’ transition by Donovan et al. There is clearly more than one band present in this region; none of the bands exhibit definitive signatures of ‘Z+-‘Z+ or ‘II-‘Et transitions. 4. CONCLUSIONS

A high-resolution, vacuum ultraviolet study of the CS B’F-X1X+ system has been performed. Four bands of ‘*C3*S (the O-O, l-0, 2-0, and l-1 bands) and the 1-O

VACUUM

ULTRAVIOLET

SPECTRUM

OF CS

429

bandheads of 13C32Sand ‘2C34Swere observed in absorption. A rotational analysis of the 1-O band, the only band with resolved rotational structure, was performed. The equilibrium molecular constants of the B’Z+ state are close in value to the published CS ground state constants, indicating that the B’Z+ state is a Rydberg state of the CS molecule. A number of diffuse absorption features short of the B-X system have been observed and measured. The tentative assignments of Donovan et al. have been, with a few exceptions, verified. ACKNOWLEDGMENTS The authors gratefully thank W. H. Parkinson for his comments and assistance. This work was supported in part by a grant to Harvard University by the National Aeronautics and Space Administration (NSG-7304). RECEIVED:

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