Far-infrared laser magnetic resonance spectrum of the AsH radical in X3Σ−

JOURNAL

OF MOLECULAR

SPECTROSCOPY

106, 423-429 (1984)

Far-infrared Laser Magnetic Resonance Spectrum of the ASH Radical in X3x:KENTAROU

EIZI HIROTA

KAWAGUCHIAND

Institute for Molecular Science. Okazaki 444. Japan

The far-infrared laser magnetic resonance spectrum of ASH in the ground X3Z- state has been observed using five optically pumped laser lines as sources. The ASH radical was produced by the reaction of metallic arsenic with microwave discharge products of a H2 and O2 mixture. From an analysis of the observed spectrum, the following rotational and fine structure constants were obtained: E0 = 215 877.54 (23) Do = 9.834 (1 I), X = 1 763 488 (56), and y = -8 114.5 (60), a11in MHz with 30 in parentheses. The hyperfine coupling constants were also determined for the two nuclei as follows: crAI = - 11.5 (14), flAs = -159.4 (13), eQqA, = -97.6 (72) cq, = -49.80 (66) and OH = 4.15 (60), again in MHz with 3a in parentheses. INTRODUCTION

Far-infrared laser magnetic resonance (LMR) has been applied to diatomic hydrides of the group V elements, NH (I) and PH (2, 3), to observe the pure rotational spectrum and to determine molecular constants, including hyperhne coupling constants, precisely. In the present study we have extended the measurement to the ASH radical in the ground vibronic state. Dixon and Lamberton (4) have investigated the A311j- X32- transition of ASH and AsD under high resolution, by flash-photolyzing AsH3 and AsD3, respectively. No vibrational and rotational spectra of both ASH and AsD have been reported before. We have observed five rotational transitions of ASH in the region 639-1799 GHz using a LMR spectrometer to determine molecular parameters, including hyperfme coupling constants, precisely. EXPERIMENTAL

DETAILS

The far-infrared LMR spectrometer employed in the present study was described in detail in Refs. (3, 5). Table I lists the CO2 laser pumped far-infrared laser lines used as sources. The ASH radical was generated by passing microwave discharge products of a H2 and O2 mixture over metallic arsenic powder contained in a glass boat, which was placed near the center of the pole caps. The partial pressures were typically 160 and 10 mTorr, respectively, for Hz and 02, and the mixture was pumped with a speed of about 2 liters/set through a cell by a mechanical booster pump. Oxygen was indispensable to initiate the reaction generating ASH, but the ASH signal was later found to be sustained even without oxygen. The resonance magnetic field was measured in the following manner. The magnetic 423

OO22-2852184$3.00 Copyright 0

1984 by Academic Press, Inc.

All rights of reproduction in any form reserved.

424

KAWAGUCHI

AND

TABLE Far-Infrared Lasing

CD2

HIROTA

I

Laser Lines Used in the Present laser

wavelength

line

gas

Study Frequency

cum)

(GHz)

CH3OH

lOR(38)

469.0

639.1846

a

CH30D

9R(8)

294.8

1016.8972

b

CH30H

9R(lO)

232.9

1286.9995

=

CH2F2

9R(34)

214.6

1397.1186

=

CH2F2

9R(22)

166.7

1798.6470

'

a.

F. R. Petersen, K. M. Evenson, D. A. Jennings, J. S. Wells, K. Goto, and J. J. Jimeney, IEEE J. Quant, Elect. QE-11 838842 (1975).

b.

T. G. Blaney, D. J. E. Knight, Commun. 25, 176-178 (1978).

C.

and K. F. R. Petersen, A. Scalabrin, Infrared Millimeter Waves 1, 111-125

and E. Murray-Lloyd,

Evenson, (1980).

M.

Opt.

Int. J.

field was first set nearly at the center of eight hyperfine components of each MJ transition, and was then scanned over flO0 Gauss around the central value. The dial reading of the magnet power supply was calibrated at three fields, lowest, center, and highest, using an NMR gaussmeter with a probe placed between the absorption cell and the pole cap. Allowance was made for the difference in magnetic field at the position of the probe and the middle of the pole caps. The uncertainty of the measured field strength was estimated to be about +3 Gauss, which included that due to insufficient resetability and drift of the far-infrared laser frequency from the center of the gain curve. When the Zeeman coefficient of the observed transitions was used, this uncertainty in magnetic field corresponded to about 1 MHz in frequency scale. OBSERVED

SPECTRUM

AND

ANALYSIS

Each MJ transition of the ASH radical was observed to be split into eight hyperline components spread over 200 Gauss, as shown by a typical example of the IV, J, MJ = 3,4, 2 - 2, 3, 1 transition reproduced in Fig. 1; this spectrum was recorded using the CH2F2 214.6~pm laser line as a source. The signal-to-noise ratio of the observed spectrum was ten times less for ASH than for PH (3) although the reaction conditions and the laser power were nearly the same for the two molecules. Molecular constants of AsH reported by Dixon and Lamberton (4) were employed to predict the rotational energy levels, and were later found to be useful in making assignments for the observed spectrum. Figure 2 shows the rotational energy levels and also the transitions observed in the present study. The spin-spin coupling constant is much larger than the rotational constant in ASH, unlike NH and PH; in other words, ASH closely approximates Hund’s case (c) when J is small, and the Zeeman

FAR-INFRARED

LMR SPECTRUM OF ASH

u

ULILI - 112 112

M, = 3/Z I

-312

I

I

6.75

670

425

I

6.80

6.65 kGauss

FIG. 1. Observed spectrum of the N, J, M, = 3, 4, 2 - 2, 3, I transition of ASH recorded using the CH2F2 214.6-cm laser line as a source. M, denotes the magnetic quantum number of the arsenic nuclear spin angular momentum. Further doublet splitting is caused by the proton hype&e interaction.

shift is thus almost linear in the magnetic field, because the off-diagonal elements of the Zeeman term contribute little to the shift. The eight hyperfme components are ascribed to the interaction of the unpaired electrons with both nuclei, proton of Z = l/2 and arsenic of Z = 3/2, the arsenic splittings being larger than those due to the proton. In the strong field case, the hyperfme interaction energy is proportional to A4,MJ, whereas the quadrupole energy conforms to [3M: - Z(Z + 1)][3M: - J(.Z + l)]. Therefore, the hyperfine splitting due to the arsenic nucleus will show deviations from an equal interval pattern, as clearly recognized in Fig. 1. The MI assignment for the As hyperfine structure was made using hyperfme parameters estimated from those of PH. Table II summarizes the observed rotational transitions and the measured resonance fields. The Hamiltonian employed to analyze the observed spectrum is given by H = BON2- DON4+ (2X/3)(3S; - S2) + yN . S + Hz + HHFS,

GHz

(1)

cm-l

6ooor-Y200 1

-50

i 3 il5pm

-0

--50 N=O

1

2

3

4

FIG. 2. Rotational energy levels of the ASH radical in the ground vibronic X’Z- state. Arrows indicate the transitions observed in the present study.

426

KAWAGUCHI

AND HIROTA

TABLE II Observed Far-Infrared LMR Transitions of the ASH Radical Laser frequency

Field N,J

MJ

obs (G)

(GHz) 639.1846

1,2+0,1

-1+-O

-2f-1

1016.8972

1286.9995

1397.1186

2,3+1,2

3,3+2,2

3,4+2,3

0+-l

1+2

1+0

2'1

3+2

4+3

-3+-3

1798.6470

4,5+3,4

l+O

2+1

3+2

Hyperfine

Splitting(H)

MI(As) c-0

-

(MHz)

1.5 0.5 -0.5 -1.5

8190.8 8212.2 8255.7 8313.9

0.6 -0.7 -0.1

1.5 0.5 -0.5 -1.5

9594.3 9628.6 9673.2 9722.8

1.5 0.5 -0.5 -1.5

obs (G)

-

c-o (MHz)

-1.7

18.8

0.0

0.2 1.2 0.1 0.5

18.4

0.3

12516.9 12550.3 12593.8 12644.2

0.0 -0.3 0.5 -0.2

19.1

0.1

-1.5 -0.5 0.5 1.5

10741.4 10888.4 11014.1 11119.2

-0.1 0.0 0.4 -0.3

14.5

0.0

1.5 0.5 -0.5 -1.5

5661.4 5706.4 5749.8 5790.5

0.3 -0.0 0.1 0.3

19.3

0.1

1.5 0.5 -0.5 -1.5

6721.4 6774.8 6819.0 6854.6

-0.6 -0.6 -0.8 -0.6

19.0

0.0

1.5 0.5 -0.5 -1.5

8263.9 8321.2 8366.7 8401.7

0.6 0.6 0.3 0.5

19.1

0.0

1.5 0.5 -0.5 -1.5

10669.1 10723.9 10773.3 10816.8

-0.4 0.0 0.4 0.5

19.3

0.1

1.5 0.5 -0.5 -1.5

12024.2 12034.2 12059.5 12142.6

-0.6 0.9 -0.1 -0.1

19.1

0.0

1.5 0.5 -0.5 -1.5

7841.2 7887.8 7931.3 7974.0

-0.0 0.4 0.2 0.5

18.8

-0.1

1.5 0.5 -0.5 -1.5

9209.2 9261.4 9307.7 9344.3

0.2 0.3 0.7 0.2

18.9

-0.1

1.5 0.5 -0.5 -1.5

11141.9 11200.9 11246.5 11283.0

-1.0 -0.4 -0.7 -0.4

19.7

0.2

FAR-INFRARED

LMR

SPECTRUM

427

OF ASH

where HZ and HHFsdenote the Zeeman and hyperfine Hamiltonians form:

of the following

and HHFs = a&I - S) + p,,(3Z&

- I - S) + eQqA,(3Zf - 1’)/[41(2Z - l)].

(3)

In Eq. (2) ,f3denotes the Bohr magneton, and the two hyperfine parameters, (Y,&and = bAs + c&/3 and PAS, in Eq. (3) are related to the Frosch-Foley parameters as (Y.&s PAS= cAJ3. A Hamiltonian similar to Eq. (3) without the eQq ttXIII appkS to the hyperfine interaction for the proton. Matrix elements of the total Hamiltonian Eq. (1) were evaluated by the irreducible tensor method (6) using Hund’s case (b), decoupled wavefunctions as bases, and the matrix thus obtained was truncated at AN = +2 and was numerically diagonalized. The least-squares analysis was carried out in two steps. In the first step, the average frequencies of the hydrogen hyperfine doublets were employed to determine molecular constants, except for the proton hyperfine coupling constants. The second step was devoted to the analysis of the proton hyperfme doublet splittings. Molecular parameters thus obtained are listed in Table III; the standard deviations of the fits are 0.6 and 0.4 MHz, respectively, for the first and second steps. The centrifugal distortion terms for the spin-spin and spin-rotation interactions could not be determined, because only five rotational transitions were observed and the applied magnetic field was insufficient to mix two levels of J different by + 1. DISCUSSION

Molecular constants obtained by the present study are compared, where possible, with those reported by Dixon and Lamberton (4). in Table III. The two results agree within three standard errors of the latter. The magnetic hyperhne coupling constants of ASH are determined for the first time. Table IV lists the results with those of NH and PH for comparison. It is interesting to note that the As Fermi contact term or the s character of the unpaired electron orbital around the As nucleus is negative, in sharp contrast with the two other cases. This fact may be accounted for by the pseudo-contact interaction, which is caused by interactions with excited electronic states through the I - L hyperhne and L - S spin-orbit interaction terms. The spin-orbit interaction also makes most dominant contributions to the X spin-spin interaction constants of ASH and PH, but plays only a minor role in NH (7). The unpaired electron spin density at the As nucleus was calculated from PASby comparing it with the corresponding atomic value (8) where the angular part in ((3 cos’x - l)/r3),” was assumed to be -215. As shown in Table IV, the spin density increases on going from NH to ASH. The observed As nuclear quadrupole coupling constant corresponds to ((3 cos’x - l)/r3)r of -9.66 X 1O24cmm3, whereas the @Ascoupling constant leads to the unpaired electron spin average ((3 cos2x - l)/r3)u of -23.5 X 1O24cm-3, where the suffixes Tand U refer, respectively, to the averages over the total electron and unpaired electron distributions. The latter value gives a rough estimate for the contributions of the ‘lr(unpaired) electrons to the eQq constant, namely, +237.1 X 2 = +474.1(39) MHz, where the factor 2 accounts for the contributions of the two unpaired electrons

428

KAWAGUCHI

AND HIROTA

TABLE III Molecular Constants of the ASH Radical in the X32- State’

constant

present

BO

b

215 877.54

DO

(23)

9.834

A

1763 488

Y

0.000620

gr

(60)

'As

-11.5

(14)

BAS

-159.4

(13)

eQqAs

-97.6

(72)

aH

-49.80

(66)

6,

4.15

(60)

1764 818

(450)

-8 424

(150)

d (78)

a.

In MHz,

b.

Values in parentheses of the constants.

c.

Ref. (4). Values in parentheses are errors given original paper; they seem to represent one 0.

d.

Fixed

to

except

(3)

(33)

[-0.018781

*g'

215 845

9.9

(60)

2.01598

%+gQ

c

(11)

(56)

-8 114.5

0pt1cal

for the g factors. denote

30 and apply

to the last digits

in the

Ag’2-gr=+Y/(2B).

occupying the rr, and ?T,,orbitals separately. The contribution of the u electron to the eQq constant is then calculated by subtracting +474.1 MHz from the observed eQq value, namely -571.7(82) MHz. Gordy and Cook (9) list the quadrupole coupling constant due to one atomic p electron in the (n10) orbital, eQqnlo; the value for As is roughly estimated to be -400 MHz. However, a more reasonable value may be obtained from ( l/r3) = 9.102 reported by Morton and Preston (8) for As. If this value is used instead of Gordy and Cook’s 7.32, eQqdlo is changed to -497 MHz. The u electron contribution of -57 1.7 MHz is thus 15.0% larger than leQq4101,which suggests the s character of the (r bond to be about 15.0%. The ionic character of the s bond is perhaps small, because the electronegativity of As is very close to that of H. It is difficult to estimate the d character. Dixon and Lamberton (4) have calculated the internuclear distance (the so-called r. value) from the observed B. constant to be 1.535 and 1.5306 A, respectively, for ASH and AsD. The present B. constant of ASH gives an essentially identical result,

FAR-INFRARED

LMR SPECTRUM

429

OF ASH

TABLE IV Comparison of the Magnetic Hyperfme Coupling Constants of the XH Molecules (in MHz)

constant

NH a

19.22

“X

(s

cnaracter,%)d

PH b

127.5(23)

(18

(1.1)

(spin density,%)d

-157.6

(82)

%H

-66.23(32

BH

32.56(53

-11.5(14)

(0.96)

-22.64(20

8,

ASH '

(-0.08)

-159.4(13)

(29)

(86)

(96) -49.80

-47.91(84)

5.6 (10)

a.

F. C. "a" den Heuvel, W. L. Meerts, Phys. Lett. 92, 215-218 (1982).

b.

Ref.

c.

Present

d.

Atomic

(66)

4.15(60)

and A. Dymanus,

Chem.

(3). _ study. values

reported

in Ref.

(8) _ were

used.

1.5343 A (there seems to be a small difference in conversion factors). In a previous study on PH (3) we have estimated three kinds of corrections to obtain the BomOppenheimer equilibrium distance reB” . When the ratio rf”/rO of PH is simply transferred to ASH, the rF” distance of ASH is calculated to be 1.522 A. This value is slightly larger than the ASH distances in AsH2 (rO = 1.5 18 A) (10) and AsH3 (rC = 1.5 108 A) (II). A similar treatment is also applied to the centrifugal distortion constant to estimate the harmonic frequency w, to be 2183 cm-‘. ACKNOWLEDGMENT Calculations in the present work were carried out at the Computer Center of the Institute for Molecular Science. RECEIVED:

March 15, 1984 REFERENCES

1. F. D. WAYNE AND H. E. RADFORD,Mol. Ph.vs. 32, 1407-1422 (1976). P. B. DAVIES,D. K. RUSSELL,D. R. SMITH, AND B. A. THRUSH,Cunad J. Phys. 57,522-528 (1979). N. OHASHI,K. KAWAGUCHI,AND E. HIROTA,J. Mol. Specfrosc. 103, 337-349 (1983). R. N. DIXON AND H. M. LAMBERTON,J. Mol. Specwosc. 25, 12-33 (1968). K. KAWAGUCHI,S. SAITO,AND E. HIROTA,J. Chew. Phys. 79,629-634 (1983). 6. I. C. BOWATER,J. M. BROWN, AND A. CARRINGTON,Proc. R. Sot. Lo&. A 333, 265-288 (1973). 7. M. HORANI, J. ROSTAS,AND H. LEFEBRE-BRION, Cunad. J. Phys. 45, 3319-3331 (1967). 8. J. R. MORTON AND K. F. PRESTON,J. Mag. Rex 30, 577-582 (1978). 9. W. GORDY AND R. L. COOK, “Microwave Molecular Spectra,” Wiley-Interscience, New York, 1970. IO. R. N. DIXON, G. DUXBURY, AND H. M. LAMBERTON,Proc. R. Sot. Land. A 305,271-290 (1968). II. W. B. OLSON. A. G. MAKI, AND R. L. SAMS,J. Mol. Spectrosc. 55, 252-270 (1975).

2. 3. 4. 5.