Laser spectroscopic study of the A 4∏−X 4∑− transition of MoN

J. Quanr. Speewosc.Radiar. Transfer Vol. 52, No. 2. pp. 145-I 50. 1994 Copyright 0 1994 Elsevier Science Ltd

Pergamon

Printed’;” &ea; Britain. All rights reserved 0022-4073/94 $7.00 + 0.00

00224073(94)00049-2

LASER

SPECTROSCOPIC STUDY OF THE TRANSITION OF MoN

A41-I-X4X-

N. S.-K. SZE and A. S.-C. CHEuNGt Department

of Chemistry,

The University of Hong Kong, Pokfulam Road, Hong Kong (Received 31 January 1994)

Abstract--A

41FX4C-

transitions,

(0,0),(0, I) and (0,2) of MoN between

been studied using wavelength-resolved

laser-induced

5990-6330 8, have

fluorescence spectroscopy. MoN was

produced by reacting vaporized MoCI, with microwave discharged N, gas. The ground state X4x- vibrational constants w, and w,x, of different isotopic molecules have been determined. w, (cm-‘) is 1059.2, 1057.0, 1055.8, 1053.7, 1052.5, and o,x, (cm-‘) is 4.8, 4.7, 4.7. 4.5, 4.4, for 92MoN, 94MoN, 96MoN, 9*MoN, ‘OOMoN, respectively.

1. INTRODUCTION Even though the gas phase optical spectrum of MoN has been recorded as early as in 1965,’ it was only recently that high resolution spectra were obtained and rotational analyses performed.2ms The ground state of MoN has been identified in an Ar matrix by Bates and Gruen4 as 4Xm with w, = 1040 cm-‘. This assignment of the ground state has been confirmed by Knight et alh using electron spin resonance matrix-isolation spectroscopy. The theoretical calculation of Allison et al’ is also consistent with this assignment. The orange band system, A 4H-X4Xtransition, has been recorded in emission and analysed rotationally by Carlson et a1.3 They photographed and analysed the A 4H-X4Em (0,O) band of isotopically enriched 94MoN, which yielded rotational and spin constants of both the A 411 and X4X:- states. Natural molybdenum has seven isotopes with atomic numbers (and abundances) 92 (15.84%), 94 (9.04%), 95 (15.72%), 96 (16.53%) 97 (9.46%) 98 (23.78%) and 100 (9.63%). Using intracavity laser spectroscopy, Pazyuk et al8 have determined the 98MoN vibrational frequency AG,,, of 4YZ3,2 and 4E,,2 to be 1044.5 and 1045.3 cm-‘, respectively. Berezin et al4 analysed low-resolution pulsed dye-laser-induced fluorescence spectra and obtained the ground state vibrational constants with o, = 1056.97 cm-’ and o,x, = 5.59 cm-’ for 98MoN. We report here wavelength-resolved laser-induced fluorescence (LIF) studies of the A 4IIX4Xm transition with the aim of measuring the vibrational frequencies of the ground 4Z- state directly. Spectra of the five isotopic molecules 92MoN, “MoN, 96MoN, 98MoN, ‘OOMoN have been obtained and analysed, and their ground state vibrational constants have been derived.

2.

EXPERIMENTAL

DETAILS

MoN was produced by reacting microwave-discharged (2450 MHz) N, with vaporized MoCl, in a Broida type oven.’ The solid MoCl, used has the natural abundances of the seven MO isotopes. Figure 1 shows the reaction zone where the activated N, is confined to react with MoCl, vapour at the tip of the oven. This reaction produces a light grey chemiluminescence. The wavelengthresolved fluorescence experiments were performed using a commercial tunable ring dye laser with rhodamine 590 and 610 dyes and pumped by an argon ion laser. Most of the laser output was sent in a single beam to the oven which was fitted with Brewster-angle windows. The laser beam was tTo whom

all correspondence

should

be addressed. 145

N. S.-K. SZE and A. S.-C. CHEUNG

146

I-. I

I

I I I

I I

1 \

1 1

I I

MoN Chemiluminescence

Alumina crucible with MoCI,

Fig. I. Schematic

diagram

of the oven system used in the production

of MoN.

passed vertically through the reaction zone into a crucible containing solid MoCl,. The laser beam was chopped mechanically at about 1800 Hz to improve the signal-to-noise ratio of the excitation spectra. and the resulting fluorescence was detected with a photomultiplier tube whose output was fed into a lock-in-amplifier. Doppler-limited spectra were recorded over selected frequency regions between 15,800-16,700 cm-‘. Frequency calibration was obtained by inserting two beam splitters in the path of the output of the dye laser to produce two extra beams. One of the beams was passed through a Fabry-Perot etalon with free spectral range of 300 MHz to generate a series of equally spaced fringes. The other beam was sent through a cell containing I?. The wavenumbers of the lines of Z2 are listed in Gerstenkorn and Luc.” In the wavelength-resolved fluorescence experiments the laser frequency was tuned to selected rotational lines, and laser-induced fluorescence was focused using a system of lenses onto the slit of a 0.75 m focal length monochromator. The dispersed fluorescence was detected by a photomultiplier tube mounted at the exit slit of the monochromator. The optimum or working resolution of the wavelength-resolved fluorescence spectra was about 0.3 cm~ ‘. The monochromator was calibrated using emission lines from an argon discharge.”

Laser spectroscopic

3.

study of the A 4CX4Z-

RESULTS

AND

transition

of MoN

147

DISCUSSION

The laser-induced fluorescence (LIF) spectrum of the A 4H-X4C- (0,O) transition of MoN between 5990-6330 8, has been recorded with Doppler limited resolution. Both the A 4H and X4X,states are close to Hund’s case (a) coupling,3 the gross appearance of the system shows four strong sub-bands at 5996, 6123, 6245 and 6305 A which correspond to 4H5,2-4X$24H3,2-4X~zand 4rI _,,z-4&$ transitions. The observed band system is complicated and shows the combination of high-spin multiplicity transitions and multiple isotopic sources. We have searched for branches that are subjected to minimum overlapping and lines from individual isotopes are resolved for further wavelength-resolved fluorescence work. Figure 2 shows the Q, (22.5) lines of the 4H-4C;, sub-band with five isotopic molecules clearly resolved. The strongest line in this spectrum is due to 98MoN which is the most abundant isotopic molecule. Even though the abundances of 95Mo and 97Mo are comparable to those of the other isotopes, their transitions do not appear as strong lines in the spectrum. Because 9sMo and 97Mo have both nuclear spin Z = 5/2, the transition intensity splits among the six hyperfine component&’ and is hidden underneath those strong unsplit lines. The wavelength-resolved fluorescence technique allows ready assignment of spectral lines.‘* Using this technique, we have confirmed the branch and rotational quantum number assignments of Carlson et al.’ Since both the A 4H and X4C- states are good case (a), the transitions reflect the selection rules AC = 0 and An = 0, & 1 are always stronger than those with AZ = + 1 and AR = 0, + 1. Figure 3 shows the energy level diagram of the “H,,,(a)-“X(a) subband with branch structure. The 4H,,2-+ 4C,i2 transitions are stronger than the 411,,2+ 4Xu2transitions. The wavelength-resolved fluorescence lines from the 4H,,2 to 4Ci2 substate always show the expected PQR pattern, however, it can be easily seen that the fluorescence pattern to the 4C;2 substate has pattern depending on whether the S or e level of the upper state is being populated.‘3 Figure 4 shows the observed fluorescence pattern of thef level of the upper state being populated. In this spectrum, excitation of the Q, (22.5) line gives R, (21.5) and P, (23.5) emission lines to the lower frequency side of the Q, (22.5) line, the weaker R, (21.5), Q, (22.5) and P4 (23.5) lines of the 4H-4C;2 sub-band have also been observed. Figure 5 shows the observed fluorescence pattern of the e level of the upper state being populated. In this spectrum, the excitation of the R, (16.5) lines gives P, (18.5) and Q2 (17.5) emission lines and the 4H,,-4X$2 sub-band with R, (16.5), P, (18.5) and Q4 (17.5)

MoNisotopes

mgk 100

MoN LIF 16027.594

spectrum

16027.806 16027.858 cm-’

I, fluorescence spectrum 3OOMHzmarkers

Fig. 2. Laser-induced

fluorescence

spectrum

of MoN showing sub-band.

the Q, (22.5) lines of the 4rI,,z-4Z;z

14X

i’i. S.-K Szt and A.

s -<‘. cHH:\C;

J 22.5 -

I

R3

R 1

Pl

‘?

23 21.5 22 21.5

i N

J

24 25.5 _ 23 23.5 23 24.5 _ 22 22.5 22 23 5 _ 21 21.5 21 22.5 _ 20 20.5 20 21.5 _L 19 19.5

I

-e +f

FI

+f ;4 -e '?,

I

4C,, /_

F;

+e Fl

f b,

-z F, +f 51 +e F, -f F_ -e :. +f F"

lines also appear. The wavelength resolved LIF spectrum ol‘(0. I ) and (0. 1) tranGtion\ 01‘rhc ‘II ‘Y transition have also been recorded. which exhibit exactly the same patterns as described. At least 45 lines of the (0. 0) band of each isotopic molecule have been studied. They belong to four subband systems: ‘rIj 2m‘C, 2‘ ‘n, J ‘2, :, ‘n, T ‘C, 2 and 'Fl , 2 ‘Z, :. All resolved Huorescencc spectral lines studied are with (0. I ) transition and only two-third of the lines with (0. 3) transition were recorded, because the (0. 3) transitions are much weaker than the (0. I) transition\ due 10 unfavourable Franck&Condon factors. Molecular constants and line positions reported previouhl> were very useful in assigning the observed spectrum. The rotational Hamiltonian matrix elements of the 411 and “C states used in this analysis are the same as those by Carlson et al.’ The rotational contribution of each J level for both the upper and lower states were calculated. The vibronic origins of I’ = 1 and 2 of the X’E state were obtained by substracting the rotational contributions

Laser spectroscopic

study of the A 41FX4Z-

3.0

I

2.5 *g

transition

2.0 -

I

- 4h2

4%2

149

of MoN

4b2

- 4%2

w2.5)

;

1

8 g

1.5

-

2

R&21.5)

.z?

1.0 -

f E

0.5 -

0.0 -

-.4-J I

6225

I

6250

6275

6300

Wavelength/ii Fig. 4. Wavelength-resolved

fluorescence spectrum of ““‘MoN produced of the 411,,2-4C;Z transition.

by excitation

of the Q, (22.5) line

from the measured line frequency. The vibrational constants obtained for the X4X- state of the five isotopic molecules are listed in Table 1. The results of Berezin et al4 is also listed for comparison. The experimental conditions, in the present experiment, including higher signal-tonoise ratio on spectral lines and better spectral resolution should give more accurate measurements. Pazyuk et al* have reported the X4X- state AG,,, of 98MoN to be 1044.5 and 1045.3 cm-’ for R = 3/2 and l/2 respectively using intracavity laser spectroscopy. Our measured AC,,, of 98MoN is 1044.7cm-’ which is in excellent agreement with the results of Ref. 8. Bates and Gruen2

3.0

2.5 -

4%2

I

I

I

- 4%

4f11,2 - 4%2

.E 5

2.0 -

R&16.5)

d2

g 5 E Q) z

Q&17.5) P&18.5)

1

1.5

1.0 Q4U7.5) 0.5 R&18.5)

-

0.0

PJ16.5)

!I_._,;__

6225

6250

6275

6300

Wavelength//4 Fig. 5. Wavelength-resolved

fluorescence spectrum of 9HMoN produced of the 4fI,,2-4Z;2 transition.

by excitation

of the R, (16.5) line

N. S.-K. SE. and A. S.-C. CHELNG

150

Table

I. Vibrational errors.

Isotope “‘MoN “‘MoN ‘“‘MoN ““MON ‘““MoN

constants (cm ’ ) of the X’X state of MoN. Estimated which are 10 limits, are in parentheses.

Present (‘,. 1059.2 1057.0 1055.X 1053.7 1052.5

(0.4) (0.5) (0.4) (0.4) (0.6)

\tud) (‘J I. 4.x (0.3) 4 7 (0.3) 4.7 (0.3) 4.5 (0.2) 4.4 (0.3)

<‘alculated (‘, (‘I.,\ 1058.0 1056.5 10.1

45 3.5 45

1052.4

4.5

Ref. 4

(‘,

I’, \

1057.0 (0.5)

5 51 ((Et

determined using matrix isolation spectroscopy the CU,of the ground matrix shifts” are taken into account our gas phase measurements

‘. When

state to be 1040cm agree

well

with

the

matrix

results. The isotopic

species have the vibrational constants which are approximately related as follows: ’ ’ (u, = u L//I. q..~, = OJ,.Y L//J ‘. where /) = (/L//L I)” “. if ~1 and 11’ ‘Ire the reduced masses of th’i molecule and of its isotope. Since qXMoN is the tnost abundant isotope. the isotopic effects are calculated relative to it as tabulated in Table 1. The experimental and calculated values agree well. In our wavelength-resolved LIF spectrum of the “n, 2 ‘C, 2 subband, extra lines have been obtained. which indicates that strong perturbations exist in this component of the ‘n states.‘.” A ~hno,r~l~,rl~c,,~lc~,lr s We would llke to thank Professor 7. C‘. Steimle for prowding the molecular constants crl both .1 ‘II and X”Z; states prior tc> publication. A.S.X.C. would like to thank the Conference and Research Grants Committee, the Leung Kau Kiu Research & Teaching Endowment Fund of the University of Hong Kong for support. and the help 01 the University Workshop for the design and construction of the oven. N S -K. S. thanks the Croucher Foundation for the studentship

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. IO. I I. 12. 13. 14. 15.

J. C. Howard and J. G. Conway, ./. c#Ic~~?I. I’h~s. 43, 3055 ( 1965). J. K. Bates and D. M. Gruen J. tt~o/(,(.. .Spc~~~ro.s~~. 78, 284 (1979). R. C. Carlson. J. K. Bates, and T. M. Dunn. .I. r>~olec,.Sp~~ctrosc. 110, 215 (1985). A. B. Berezin. S. A. Dmitruk. and D. J. Kataev. 0p/. Sprctrosc.. 68, 310 (1990). D. A. Fletcher. K. Y. Jung. and T. C. Steimle. J. c~hem. fV7y.s. 99, 901 (1993). L. B. Knight and J. Steadman, J. chcrn. Ph~x. 76, 3378 (1982). J. N. Allison and W. A. Goodard III. .I. c,/zc~n~.PI7~ts. 81, 263 (19X3). E. A. Pazyuk. E. N. Moskvitina. and Yu. Ya. Kuzyakov. Sp(,(,r~.o.s(,. 1.(,[1. 21, 447 (19X8). J. B. West, R. S. Bradford. J. D. Eversole. and c‘. R. Jones. Rev. .S&nr. hstrurn. 46, 164 ( lY75). S. Gerstenkorn and P. Luc. .1 tltr.s tkc~rSpc,cfr.e c/‘uhsorpfiot~ &TIu rnoi~~c~ul~~ d’iodc, Editions du CNRS. Paris. France (1978): S. Gerstenkorn and P. Luc. Rcr. Pl7~3. A/y/. 14, 791 (1979). A. A. Radzig and B. M. Smirnov. Rc~frrcww Lkrrcr on Atoms, ~Moltwkv rrnd lam. Springer, Berlin ( 1985) W. Demtr6der. Lnsc~ S/~c,c.fr.o.sc.o/,l.,p. 41 X, Springer. Berlin (I 9X2). J. M. Brown. J. T. Hougcn, K. P. Huber. J. W. C. Johns. I. Kopp. H. Lel’ebvrc-Brmn. A. J. Merer. D. A. Ramsay. J. Rostas. and R. N. Zare. J. n&c,. Spec~/ro.rc 55, 500 (1975). J. K. Bates and D. M. Grucn. Hi,@ Tw~p. Sc,i. 10, 27 (1978). G. Herzberg. .Qczc.tr.tr of Dirr/omic~ ,2ilolc~~d~~.c. 2nd Edn.. p. 141. Van Nostrand. New York. NY ( 1950).