R. S. Ram et al- Fourier Transform Emission Spectroscopy of the A^2-Delta-X^2-Pi Transition of SiH and SiD

8/3/2019 R. S. Ram et al- Fourier Transform Emission Spectroscopy of the A^2-Delta-X^2-Pi Transition of SiH and SiD

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Fourier Transform Emission Spectroscopy of theA2X2 Transition of SiH and SiD

R. S. Ram, R. Engleman Jr.,1 and P. F. Bernath2

Department of Chemistry, University of Arizona, Tucson, Arizona 85721

Received January 13, 1998; in revised form March 26, 1998

The emission spectra of the A2X2 transition of SiH and SiD have been observed at high resolution using a Fouriertransform spectrometer. The molecules were excited in a Si hollow cathode lamp by passing a discharge through a mixture ofNe and a trace of H2 or D2. The present data, combined with the previous infrared vibrationrotation measurements, have beenused to determine improved molecular constants for the ground and excited states of SiH and SiD. 1998 Academic Press

INTRODUCTION

SiH is a free radical of fundamental importance. In recentyears, there have been numerous experimental and theoreticalstudies of SiH because of its importance in astrophysics (1) andchemical vapor deposition of thin films (2 4). Because ofsignificant cosmic abundances of Si and H, there is a strongpossibility that the SiH radical may be found in the interstellarmedium and stellar atmospheres (5). SiH has already beenidentified in the spectra of sunspots (6 8). As a chemicalintermediate this radical plays an important role in many in-dustrial processes such as plasma vapor deposition, thin filmformation, and semiconductor manufacturing. The primary di-agnostic of silane plasmas has been the observation of theA2X2 transition of SiH by optical emission spectroscopy(913), laser spectroscopy (14, 15), and laser optogalvanicspectroscopy (16).

The emission spectrum of SiH has been known since 1930when Jackson (17) observed a strong transition of SiH near 410nm using an arc source. He obtained a rotational analysis of the00 band and assigned it as the A2X2 transition. Thisanalysis was later modified by Mulliken (18). The spectra ofSiH and SiD were reinvestigated by Rochester (19), Douglas(20), and Klynning and Lindgren (21). Rochester (19) obtainedan analysis of the 0 0 and 11 bands of SiH and the 0 0 band

of SiD, whereas Douglas (20) observed the SiH bands at ahigher dispersion and analyzed 0 0, 1 0, 21, and 22 bands.In a more recent study Klynning and Lindgren (21) provided ananalysis for 11 band of SiH and the 00, 11, 22, 10, and21 bands of SiD. Several higher-lying excited electronicstates, B2, C2, D2, and E2, have also been observedfor SiH and SiD (2224). The B2 and D2 states were found

to be strongly predissociated (22, 23). More recently, a newelectronic state of SiH and SiD was detected near 46 700 cm1

by resonance-enhanced multiphoton ionization spectroscopy(25). This state was tentatively assigned as a 2 state based ona computer simulation of the spectrum.

Because of astrophysical interest in SiH, transitions inother spectral regions have also been investigated exten-sively. The lowest pure rotational transition lies outsideof the spectral range available to radio astronomers, butlambda doubling transitions can be used to detect SiH.Similar radiofrequency transitions have been successfully usedfor the detection of OH and CH in the interstellar medium (26,27). Wilson and Richards (28) obtained a -doubling fre-

quency of 2940 300 MHz for J 0.5 by extrapolating the-doubling from high rotational levels populated in the opticalexperiment of Douglas and Elliott (5). Klynning et al. (29)remeasured their high dispersion plates in order to obtain themore precise values of the -doubling constants for the ground

X2 state. From these measurements a value of 2968 6 MHzwas predicted for the -doubling transition. Cooper and Rich-ards (30) also carried out an ab initio calculation of thisfrequency. Freedman and Irwin (31) have refitted the linepositions of the 00 and 10 bands of Douglas (20) withoutfirst deriving term values [direct approach of Zare et al. (32)]

and provided a value of 2932 MHz.The SiH vibrationrotation bands were observed by Knightset al. (33) in an emission study of a silane plasma in the18002300 cm1 region. From this study the vibrational androtational temperatures of 2000 and 485 K were obtained. Theinfrared spectra of SiH were also observed by Brown andRobinson (34), Brown et al. (35), Davies et al. (36), andBetrencourt et al. (37). Brown and Robinson (34) detected thefundamental 10 band of SiH by laser magnetic resonancespectroscopy, while Betrencourt et al. (37) observed the 10,21, and 32 vibrationrotation bands at a resolution of 0.005

1 Present address: Department of Chemistry, University of New Mexico,Albuquerque, NM 87131.

2 Also at Department of Chemistry University of Waterloo, Waterloo, On-tario, Canada N2L 3G1.

JOURNAL OF MOLECULAR SPECTROSCOPY 190, 341352 (1998)ARTICLE NO. MS987582

3410022-2852/98 $25.00

Copyright 1998 by Academic Press

All rights of reproduction in any form reserved.


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cm1

using a Fourier transform spectrometer and providedmuch improved constants for the ground state. In the mostrecent study of this radical, Seebass et al. (38) have measuredseveral vibrationrotation lines in the 1 0, 21, 32, 4 3, and54 bands of five isotopomers of SiH and have determined avery precise set of ground state molecular constants includingthe spin-orbit constant of A

0 142.8957 (8) cm1.

The intensity of the A2X2 electronic transition of SiH is ofconcern to astronomers and to material scientists. The absorptioncross section and electronic transition moment of the A2X2transition have been determined by Park (39) using a shock tubeabsorption technique, and a transition moment of 0.12 0.04 auhas been obtained. The A2X2 transition of SiH has also beenobserved in vacuum UV photolysis experiments of silane (SiH

4)

and dichlorosilane (SiH2Cl

2) by Washida et al. (40). In a recent

paper Stamou et al. (41) have simulated the rotational intensitydistribution of the 00 band of the A2X2 transition of SiHobserved in emission from a radio frequency discharge and pro-posed a new set of molecular constants. There are several studiesof the radiative lifetimes of the A2 state of SiH (7, 8, 4245)using the solar spectrum (7, 8, 41), electron bombardment (4345), and laser-induced fluorescence (14, 46). The solar values,ranging from 569 to 3200 ns, have been estimated from the

oscillator strengths f00 and are less reliable. The other experimen-tal values range from 518 to 700 ns. A recent value of 534 23ns has been obtained using the laser-induced fluorescence tech-nique (46). There are several calculations providing the estimatesof dissociation energy of SiH between 3.0 to 3.35 eV. An upperlimit of 3.06 eV has been obtained from the analysis of predisso-ciation in the 2 state (22), while a value of 3.34 eV wasobtained from an analysis of the fluorescence lifetimes of the v 1 rotational levels of the A2 state (45). A summary of theexperimental and theoretical heats of formation of SiH are tabu-lated by Jasinski et al. (2).

There are several theoretical predictions of the spectro-scopic properties of the SiH radical (4754). A dipolemoment of 0.118 D was predicted by Petterson and Lang-hoff (47), in excellent agreement with the calculated valuesof Lewerenz et al. (48) (0.124 D) and Meyer and Rosmus(49) (0.115 D). Park and Sun (50) have applied the valenceshell Hamiltonian method base on quasi-degenerate many-body perturbation theory to calculate the molecular proper-ties including the valence state energies. Theoretical calcu-lations have also been focused on the study of geometricalstructures, force constants, and vibrational spectra and heatsof formation of silane (SiHn) and chlorinated silane mole-cules (SiHnClm) (5155).

EXPERIMENTAL

The SiH and SiD molecules were made in a demountablesilicon hollow cathode lamp (56). The cathode was prepared byinserting a solid rod of boron-doped silicon into a hole in a

copper block. The central part of the rod was then boredthrough to provide a uniform layer of silicon inside the cath-ode. The addition of 0.02% of boron to the silicon provided amaterial with good electrical conductivity. The lamp was op-erated at 470 V and 610 mA current with a slow flow of neon.Strong SiH bands were initially observed using neon carriergas at 2 Torr without any added hydrogen. The hydrogenapparently came as impurity in the neon gas or in the vacuumsystem. The addition of more hydrogen did not increase theintensity of SiH bands. The SiD bands were observed byadding a trace of D

2gas to the neon flow.

The spectra were recorded using the 1-m Fourier trans-form spectrometer associated with the McMath-Pierce SolarTelescope of the National Solar Observatory. The spectra inthe 17 000 35 000 cm1 region were recorded using Siphotodiode detectors and a CuSO

4filter. The SiH bands

were recorded at 0.016 cm1 resolution after coadding fivescans in about 75 min of integration, while the SiD bandswere recorded at 0.05 cm1 after coadding 25 scans in 130min of integration.

The spectral line positions were extracted from the observedspectra using data reduction programs called PC-DECOMPand GREMLIN developed by J. Brault. The peak positions

were determined by fitting a Voigt line shape function to eachspectral feature.In addition to the SiH/SiD bands, the final spectra also

contained Si and Ne atomic lines. The spectra were calibratedusing the measurements of Ne atomic lines made by Palmerand Engleman (57). The SiH/SiD lines have widths of about0.103 0.005 cm1 and appear with a maximum signal-to-noise ratio of about 30:1 in the 00 band. Because of largewidths and weak intensity, the high-J and weaker lines appearto be diffuse and slightly distorted. Part of the strange appear-ance of the lines could be due to the presence of unresolved

FIG 1. A portion of the 00 band showing some low J lines in R11, R22,RQ

12, and sR

21branches of SiH.

342 RAM, ENGELMAN, AND BERNATH



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isotope structure for high-J lines. The absolute accuracy andprecision of the measurements of sharp and unblended lines isexpected to be of the order of 0.003 cm1. However, theuncertainty of the weaker and blended lines could be as high as0.005 cm1.

RESULTS AND DISCUSSION

The SiH and SiD bands are located in the 21 00025 000cm1 region that covers the v 0 and v 1 sequences.The bands in the v 1 sequences are very weak and could

TABLE 1

Vacuum Wavenumbers (in cm1) of the A2X2II System of SiH

343THE A2X2 SYSTEM OF SiH AND SiD



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not be measured. In the v 0 sequence the 00 band is thestrongest and the 11 band is about 20% of the intensity of the00 band. In this analysis we have only included the lines ofthese two bands. The 22 band is very weakly present in our

spectra and a few Q-branch lines were seen near the heads, butthe data were not sufficient for a meaningful analysis.

In a 22 transition there are six strong main branches, P1,

P2, Q

1, Q

2, R

1, and R

2, each of which is doubled by lambda

TABLE 1Continued




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doubling in the2

state. These are the only branches that areallowed if both states obey pure Hunds case (a) coupling or ifboth states have pure Hunds case (b) coupling. For SiH,however, the A2 state changes from approximately Hundscase (a) to Hunds case (b) as J increases and several satellitebranches become allowed. In total six additional satellitebranches (QR

12, PQ

12, OP

12, SR

21, RQ

21, and QP

21) are expected

in each band. In the 00 band all of these branches have beenidentified. In the 11 band some of the satellite branches, suchas OP

12and QP

21, could not be identified because of their very

weak intensity. In SiD some of the satellite branches also werenot observed in the 00 and 11 bands. Although Klynningand Lindgren (21) observed more bands in their study, theFourier transform measurements are expected to be more pre-cise by about an order of magnitude. A part of the 00 band ofSiH showing some low J lines of a few branches has beenprovided in Fig. 1. As shown, the -doubling is resolved evenfor the lowest J lines observed. The observed line positions ofSiH and SiD bands have been provided in Tables 1 and 2.

In order to determine the rotational constants, the observed linepositions of different bands were fitted with the effective N2

Hamiltonian of Brown et al. (58). The 2 matrix element of thisHamiltonian is provided by Amiot et al. (59), and those for a 2

state are provided by Brown et al. (60). The lines in each of thevibrational bands were initially fitted separately using a nonlinearleast-squares procedure. In the final fit the zero field wavenumbersof the 1 0 and 21 vibrationrotation bands calculated by Brown(61) (provided for convenience in Table 3), the infrared vibrationrotation bands of Betrencourt et al. (37), and far infrared frequen-cies calculated from the constants of Brown et al. (35) provided inthe Table 2 of Betrencourt et al. (37) were combined with ourpresent measurements. These infrared and far infrared frequencieswere weighted by the estimated uncertainties provided in theoriginal papers. The weights for the weaker and blended lines

were chosen according to the signal-to-noise ratio and extent ofblending. The constants for SiH and SiD obtained in this fit areprovided in Tables 4 and 5. The final reduced standard deviationsare 1.20 for SiH and 1.14 for SiD. For convenience, the groundand excited state term values for SiH and SiD are also provided inTables 6 and 7, respectively. The spectroscopic parameters Tv(except v 0), Av, v, Dv (except v 0), Bv, Dv, Hv, qv, qDv, pv,and pDv were determined in the ground state. The term value forthe v 0 vibrational level of the ground state (T

0) was held fixed

to zero. Except for qv, qDv, pv, and pDv, all the above parameterswere determined in the excited A2 state. The inclusion of high-Jvibrationrotation lines from Betrencourt et al. (37) is expected toimprove the determination of distortion constants in the groundstate and hence help break the strong correlations with the excitedstate. The present ground state values are in excellent agreementwith the previously reported values (3538), partly because of theinclusion of all the previous infrared measurements. In particularthe present values of G(1/2) 1971.03904(35) cm1, A

0

142.88778(39) cm1 can be compared by the recent values ofG(1/2) 1971.0413 cm1, A

0 142.8952 cm1 obtained by

Seebass et al. (38) in their laser magnetic resonance experiment.The values of G(1/2) 1970.78 cm1, A

0 142.83 cm1

were obtained by Klynning and Lindgren (21). Some of the

discrepancies are caused by our adoption of the N2

Hamiltonianrather than the energy level expressions used by Klynning andLindgren (21). It has been shown by Brown and Watson (62) thatthe ADv and v parameters cannot be determined simultaneouslyfor a 2 state. In their analysis Klynning and Lindgren (21)determined ADv (AJ in their notation) instead ofv. The separationof these two parameters is possible only by isotopic substitution,and Betrencourt et al. (37) decided to determine v rather than

ADv. In the most recent infrared study of this radical, Seebass et al.(38) observed the spectra of five isotopic species 28SiH, 29SiH,30SiH, 28SiD, and 29SiD and found an appreciable magnitude for

TABLE 1Continued




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TABLE 2

Vacuum Wavenumbers (in cm1) of the A2X2II System of SiD




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TABLE 2Continued




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the v parameter. Our results are consistent with the results ofthese two groups.

Since the ground state molecular parameters are known withexcellent precision from the previous infrared studies (3538),the main purpose of the present study was to improve themolecular constants for the excited A2 state. It is thereforemore interesting to compare the present excited state constantsfor the A2 state with the values obtained previously by Klyn-ning and Lindgren (21) and Klynning et al. (29). Our G(1/2) 1662.36001(82) cm1, A

0 3.54137(82) cm1 are signif-

icantly different from the values ofG(1/2) 1660.58 cm1,A0

3.62 cm1 obtained by Klynning and Lindgren (21). Therotational constants for the v 0 vibrational level of the A2

state from their revised analysis of the 0 0 band [Klynning etal. (29)], however, compare better with our values. In this work

they determined 0 instead of AD0 in the excited state whilestill keeping AD0 in the ground state. Their B0 7.28280(8)cm1, D

0 5.203(3) 104 cm1, A

0 3.544(2) and

0

0.0871 cm1 compare well with our values of B0

7.287599(14) cm1, D0

5.1881(13) 104 cm1, A0

3.54137(82) and 0

0.089983(99) cm1.The molecular constants of the v 0 and v 1 vibrational

levels have been used to determine the equilibrium constantsfor SiH and SiD from an exact fit. The excited state equilibriumrotational constants obtained from this work are Be 7.502718(21) cm1, e 0.215119(15) cm

1 for SiH and Be

TABLE 3

Hyperfine-Free Vibration-Rotation Lines (in cm1) of SiH and SiD (61)




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3.878272(77) cm1, e 0.076744(68) cm1 for SiD. The

values reported in parentheses are estimated uncertainties.Since these values have been determined from exact fits, theactual uncertainties in these values are expected to be muchhigher than the quoted values. The equilibrium bond lengthsfor the excited A2 state of SiH and SiD using these values are1.5197816(21) and 1.521017(15) , respectively.

In a previous study of this molecule, Verma (22) observedstrong predissociation in the B2 and D2 states. From an

analysis of these predissociations, Verma (22) estimated anupper limit of the dissociation energy of 3.06 eV for this

molecule. The lifetimes of a number of rotational levels of thev 0, 1, and 2 vibrational levels were measured by Carlson etal. (45) using the high-frequency deflection technique. Theyobserved some decrease in the lifetimes of rotational levels at

N 12 in v 1 F1

and F2

components. They found that therotational levels up to N 11 ofv 1 have average lifetimesof 594 10 ns, and for higher N this value decreases slowlyto 470 ns in F

1(at N 18) and 405 ns in F

2(at N 16).

They also found that the v 2 vibrational level has a very short

lifetime of about 160 ns. From this observation, they concludedthat the v 1 and 2 vibrational levels of SiH are affected by

TABLE 4

Spectroscopic Constants (in cm1) for the A2X2II System of SiH

TABLE 5

Spectroscopic Constants (in cm1) for the A2X2II System of SiD




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TABLE 6

Term Values (in cm1) for the Ground and Excited States of SiH




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TABLE 7

Term Values (in cm1) for the Ground and Excited States of SiD


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predissociation. We are unable to add much to these observa-tions since our 11 band is weak in intensity and the 22 bandis almost absent in our spectra.

CONCLUSION

The emission spectra of the A2X2 electronic transitionof SiH and SiD have been measured with improved precision.

The high-resolution measurements from spectra recorded witha Fourier transform spectrometer have been combined with theprevious infrared vibrationrotation measurements (3538) toextract improved molecular constants for the ground and ex-cited states of SiH and SiD.

ACKNOWLEDGMENTS

We thank J. W. Brault and J. Wagner of the National Solar Observatory forassistance in obtaining the spectra. The National Solar Observatory is operatedby the Association of Universities for Research in Astronomy, Inc., undercontract with the National Science Foundation. The research described here

was supported by funding from the NASA laboratory astrophysics program.Support was also provided by the Petroleum Research Fund administered bythe American Chemical Society and the Natural Science and EngineeringResearch Council of Canada. We thank J. M. Brown for providing us thehyperfine-free vibrationrotation line positions for SiH and SiD.

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R. S. Ram et al- Fourier Transform Emission Spectroscopy of the A^2-Delta-X^2-Pi Transition of SiH and SiD