Resonance fluorescence spectrum of nitrogen dioxide

JOURNAL

OF MOLECULAR

SPECTROSCOPY

Resonance

413-423

50,

Fluorescence

(1974)

Spectrum

of Nitrogen

Dioxide’

K. ABEL, FRED MYERS, T. K. MCCUBBIN, JR., AND S. R. POLO 10-l Ihey

BuildinK, Department of Physics, The Pennsylvania .Tta.k l:nkersily, (university Park, Penns_vlounia 16802

We report an investigation of the resonance fluorescence spectrum of NOI excited by several laser lines. Sixty transitions, mostly in YZprogressions of the ground state, have been assigned. Analysis of the spectra extend the knowledge of the ground state constants, especially of the anharmonic coefficients. It is possible to establish that transitions are occurring to a 2B~ state and 10 a 2B1 state in the same energy region.

INTRODUCTION

The visible fluorescence spectrum of NOa excited by laser lines shows under low resolution a number of sharp vibrational features in an otherwise unresolved background (I). Under higher resolution the rotational structure of these discrete features is usually resolvable into a few individual lines. The displacements of these from the exciting line correspond to frequencies of infrared transitions between ground state levels. With the help of existing infrared data (2,3,4) it is possible to identify most of the prominent lines as resonance fluorescence transitions to ground state levels whose vibrational and rotational quantum numbers can be determined without much difficulty. The analysis of the fluorescence spectra makes it possible to improve and estend our knowledge of the ground state, in particular of the anharmonicity constants. It is more difficult to obtain information about the upper electronic state both because there is usually only one (or at most a few) rotational level(s) excited by the laser line, and also because the absorption spectrum of NO2 in the visible is so complex that it has so far resisted reasonable attempts at analysis. From our present data it is not possible to determine the vibrational quantum numbers of the levels in the excited electronic state(s). However, simple consideration of the selection rules obeyed in the observed transitions allows the determination of the symmetry species of the excited electronic states. We have reported recently (5) the observation of a Ye ground state vibrational progression with excitation by the 5145 a line of an argon laser. The selection rules obeyed indicate that the transitions were of a-type (corresponding to parallel bands in 1This research received support from the National Science Foundation and Environmental Agency under Grant No. AP-18. * Present Address: National Research Council of Canada, Ottawa 7, Canada. x Present Address: Kenyatta Cottege, Nairobi, Kenya, EAST AFRICA. 413 Copyright .&lI rights

0

1974 by Academic

of reproduction

Press.

in any form

Inc. resen.ed

Protection

414

ABE ET AL. Table I Vibronic

Symmetry

A

D

Bb (00)

(ee)

"2

Species B c (06)

(eZ)

!-1 I

I

I

I

II

i

I

I

I

I

I

700

770

760

750

740

1530

I

2260

1520

I

2270

1510

I

2260

1500

cm-l

cm-1

I

2250

cm-'

FIG. 1.The ~2progression with 5145A excitation.

RESONANCE

FLUORESCENCE

SPECTRUM. OF NO,

41.5

2”2

I

I

1510

I

I

I 2250

bkG.

I

1500

I 1490

I

cm-1

‘I 2240

cm-l

2. The VPprogression with 4965 .fl excitation

the limiting symmetric-top approximation). This established the esistence of a 2B, excited electronic state in addition to the ‘RI state previously identified by Douglas and Huber (6). The present paper describes additional investigations performed with excitation by the 6328 A line of a helium-neon laser and by the 514.5 A, 4965 hi, and 4880 A lines of a new argon ion laser, more powerful and more nearly monochromatic than the one used in our previous work. The new resonance fluorescence transitions observed are again mostly ~2 (al) ground state progressions, although combinations with 2vs(aJ are also observed. In excitation by the 5145 A Iine there is a transition observed in

416

ABE ET AL.

FIG.

I

I

I

I

2260

2250

2240

2230

cm-'

3. The YZprogression with 6328 A excitation.

the anti-Stokes side of the exciting line which (together with the other members of a V~ progression) obeys c-type band selection rules (corresponding to AK = f 1, perpendicular

band selection

the excited electronic

rules in the limiting

state of these transitions

symmetric-top).

is of 2B1 symmetry

This indicates

that

species, possibly the

same found by Douglas and Huber (6). We have thus now established that transitions to this 2Br state and to the 2Ba state previously reported (5) occur in the same energy region. EXPERIMENTAL

The sample of NOn gas at room temperature and at a pressure less than 1 Torr was located in a focused beam in an optical cavity external to the laser. The fluorescence was observed using a 2.5 m doubly passed Czerny-Turner echelle spectrograph, as in our previous work (5). The argon ion laser had a line width of 0.13 cm-l, narrower than that of the laser

RESONAiTCE

FLUORESCENCE

I

I 3930

I

I

I

3210

3220

SPECTRUM

3190

3200

I

CM-’

I

I

3940

417

OF NO?

3930

3920CM-’

FIG. 4. The 2~~ anti Y? + 2~ bands with 5145 -4 excitation.

used for our previous excitation

to use narrower using

slits, given

the emission

frequencies 0.2 cm-‘,

NO:, studies

is more selective

because

and the spectra a spectral

of a Be0 plasma are cleaner

slit width

tube.4 As a consequence,

than

of 0.2 cm-‘.

before.

The calibration

was made

lines of the neon discharge.

of the emission corresponding

lines is within

to the uncertainty

escitation. The 6328 A line of a helium-neon

The accuracy in the measurement 0.05 cm- I. The precision is probably

as to which

laser mode

laser was also used.

the

It was also possible

is responsible

The output

power

of the about for the

was about

80 mW. This line falls on the wing of the NOs absorption band and the signal is weak. voltage 1800 V. The spectral slit width used was 1.0 cm-l, and the photomultiplier As a result,

the recorded

spectra

are poorer,

and

the calibration

error

perhaps

up to

0.5 cm-‘.

The KO, molecule has an odd number of electrons and a total spin of 3. In its ground electronic state the doublet splitting is small and the total angular momentum exclusive of spin, defined

as

N=J-S, is a

UJnStant

of the motion

to a very good approximation.

From this follows the selection

4 Manufactured by Carson Laboratories, Inc., Bristol, Connecticut.

418

ABE

ET AL.

PURE ROTATION

I 2070

FIG. 5. The pure rotational,

I

I

2060

2050

PI and ~2 + 2~2 spectra

,,I

with 6328 w excitation.

rule AN = AJ = 0, &l. For the most abundant isotopic species, N1602, the only levels allowed by the exclusion principle are symmetric with respect to the interchange of the identical nuclei. The relationship between the rigorous overall species classification of the levels under the permutation-inversion group (1 X P2) on the one hand, and the vibronic (C,,) and asymmetric-top (V=Dn) classifications on the other hand, is well known (7). The results are summarized in Table I. As usual, the choice of axes is a = y, b = z, c = 2, so that for the vibronically allowed transitions the symmetry species of the transition moments for a-, b-, or c-type bands are &, A1, or Br, respectively. The rigorous selection rules are +t-,-

and

SOS.

The ground state progression of bands whose displacements from the exciting line correspond to the vibrational frequencies v2, 2~ and 3~2, are observed with excitation

RESONANCE

FLUORESCENCE

SPECTRUM

OF NO?

419

by the 5145 if, 4965 ii, and 4880 A lines of the argon laser, as well as by the 6328 A line of a helium-neon laser. They are illustrated in Figs. 1, 2, and 3. Other bands having the same type of structure are those corresponding to 2~ and ~3 + 2~3 excited by the 5195 A line (Fig. 4), and those corresponding to vr and v1 + v2 excited by the 6328 A line (Fig. 5). In both cases we found that the fluorescence occurs from the same state that gives origin to the V~progression. In each band, the most prominent discrete feature is a set of three lines which are easily

interpreted

as originating

in a common

rotational

level in the excited

state,

and

ABE ET AL.

420

ANTI-STOKES BAND

OF

U2

740

750

760

770

ml-’

FIG. 6. Top : The ~2 region with 5145 A excitation. The electronic apparatus was adjusted to provide more amplification than was used for the spectrum of that region shown in Fig. 1. Bottom: The antiStokes ~2 spectrum with 514.5 8, excitation.

obeying the selection rules AN = AJ = 0, f 1. With this assumption between successive lines in each set should be given by 2&,“N

the two intervals

and 2&,“(N

+ 1) where

N is the rotational angular momentum quantum number of the upper state level (and of the middle lower state level). The ratio of these two intervals can be determined in most cases with enough accuracy to be able to obtain the value of N unambiguously. At any rate, the knowledge of even an approximate value of &,” is also sufficient to determine the value of N unequivocally. In all these bands the PQR intensity ratio establishes that the transition moment is parallel to the axis of least inertia,

the u-axis, or in the symmetric

top approximation

of type AK, = 0. The value of K can be determined because the large variation of the rotational constant A with excitation of the bending vibration, yz, makes the displacements of the lines of a progression sensitive enough to the values of K. The assignments may be confirmed by calculating the relative intensities of the lines. In the limiting prolate top approximation they depend only on N and K (8, p. 226).

RESONANCE

FLUORESCENCE

1590

1580

1570

SPECTRUM

1560

OF NOz

421

cm-'

FIG. 7. The vt band with 4880 I%excitation

As we have mentioned previously, with 514.5 A excitation a band is observed in the anti-Stokes region of the spectrum. This band is marked I in the lower part of Fig. 6. Another band with identical structure is also shown in the upper part of the figure. The fact that a band appears on the high frequency side of the exciting line is easily interpreted on the basis that the molecule was originally in the 010 excited vibrational state when the laser excitation occurred. Subsequent fluorescence emission to the 000 ground vibrational state gives rise to a band displaced by a frequency shift corresponding to v2 towards the anti-Stokes region. Emission to the 020 level gives rise to the band on the Stokes side shown in Fig. 6. For both bands the Q-line is by far the strongest, the observed intensity ratios being approximately 1.2: 2.5: 1.0, and 1.2: 2.4: 1.0, respectively. With the assigned values of S and K given in Table II the intensity ratios calculated for a perpendicular band in the symmetric-top approximation are 1.3:2.3: 1.0. This indicates that this is probably a c-type transition and that the upper electronic state is of 2Br species, and quite possibly the same state found by Douglas and Huber (6). With 5145 A excitation bands corresponding to vi, 2v,, ~1 + v2, v1 + 2v?, IQ + 3~~ were also observed, but the rotational assignments have not yet been made. The band with displacement 1570 cm-’ obtained with 4880 A excitation was observed by Sakurai and Broida (1) and assigned to v 3, although its frequency is lower than the value obtained from the infrared. The present work shows that two overlapping paralleltype bands occur in this region, as shown in Fig. 7. The discrepancy can be explained by the following interpretation. The excitation takes place to some level of the 2Bt electronic state with the v3(b3) vibration escited (an odd number of quanta) so that the excited state has vibronic symmetry il r. This is a b-type vibronically allowed transition, corresponding to a perpendicular type band in the limiting prolate-top case. The emission takes place to the 001 vibrational level (of species bz) of the ground 2A 1 electronic state. The transition moment is of species By, giving rise to an allowed u-type band, which corresponds to a parallel band in the prolate-top limit. The large value of the .4 rotational constant and the change in Kvalue on excitation account for the large shift from the expected band center.

ABE ET

422

AL

We have also found bands corresponding to ~1, v2, and 2~ with 4880 A excitation. However, the rotational assignment has not yet been made because overlapping transitions give rise to rather complicated structures. ANALYSIS

The expression for the term values can be written T = The vibrational

G(0,0,O)+

in the general form

Go(Q, Q,Q) + F,,] (N, K,, K,) + EL,] (1, Ka).

term is given by Go(vl, vz, vx) = C wiOv<+ C x;~OV,V~ + . . . . * is j

No terms higher than quadratic are needed to represent the vibrational energies in the present work. Since NO? in its ground state is a slightly asymmetric, near-prolate top, the rotational term is usually well represented by the approximation F[,J(N,

K) = B,“,N(Ai

+ 1) + (A [Ul - -&)K”

where B = (l/2) (B + C), and K = K,. Actually, the asymmetric top splitting becomes significant for the K = 1 levels, in which case the rotational energy expression has to be corrected by the addition of a term f (l/4) (B - C)N(N + l), with the upper value corresponding to the levels with K, = N - 1 and the lower to the levels with K, = N. Since B - C ‘V 0.023 cm-‘, this correction already amounts to 0.12 cm-’ for A7 = 4. As it has been already indicated, for the A r vibronic levels of the 2A1 electronic ground state only those levels are present for which K, and K, are both even or both odd. For K = 1 this implies that if N is even (odd) only the upper (lower) levels which correspond to an asymmetry pair will be present. The change in the rotational constant A with the quantum number of the bending vibration is rather large (cyzoA= - 0.36 cm-l). In order to reproduce our results to the accuracy of our measurements we have found it necessary to retain the quadratic in the expression for A tV]. For the rotational constants BI,] and Ct,] it is term ye O*V~~ sufficient to retain the terms lytO% and (~2~~7~2, respectively. The correction for spin-rotation interaction is well represented by a term of the form E[,I(J,

K,) = f

q,1Ka2,‘(2J + 11,

for each pair of levels with J = N f + respectively. For the ground electronic state the value of E[“I is approximately 0.18 cm-’ (J,9). As we have already mentioned, our measured displacements from the exciting line are affected by the uncertainty about the frequencies of the laser modes responsible for the excitation. The differences in the displacements of the lines originating in the same upper state are more accurately determined. The values of the molecular constants of NOz than can be determined more reliably from the present work are the following. xl+ = -

6.5 cm-‘,

xoP I_ = xz30 = -

0.5 cm-‘, 12.4 cm-l,

RESONANCE

FLUORESCENCE a2”A =

-

rzO” = -

SPECTRUM

OF NOz

423

0.34 cm-‘, 0.015 cm-‘.

The value .x22= - 0.24 cn-l quoted in our previous work (5) should really be xzgn + 16y20A, since rtoA is not negligible. This accounts for the apparent discrepant). with our present value of x:5$‘. The values of xIzn and xnsn differ from the values obtained by Arakawa and Nielsen (.?) but are in very good agreement with other, more recent work.: KECEIVED:

August 13, 1073 REFERENCES

1. 2. 3. -1. 7. 6. 7. S.

K. SAKUKAI AND H. P. BROIDA, J. Chem. Phys. 50, 2404 (1969). F. L. KELLER AND A. H. NIELSEN, J. CRem. Phys. 29, 2.52 (195X). E. T. AKAKAWA AND A. H. NIELSEN, J. Mol. Spectrosc. 2, 413 (19.58). M. D. OLMAN AND C. D. HAUSE, J. Mol. Spectrosc. 26, 241 (1968). K. AHE, 1:. MYEKS, T. K. MCCU~BIN, AND S. R. POLO, J. Mol. Spectrusc. 38, 552 (1971). A. E. DWCLAS AND K. P. HUUEK, Can. J. Pllys. 43, 74 (1965). K. S. MVLLIKEN, Phys. Rez. 59, 873 (1941). G. HEKZHEW, “Molecular Spectra and Molecular Structure,” Vol. 3, Van Nostrand, Princeton, 1966. Y. M. L. LEES, K. 1’. CURL, JK., AND J. G. BAKER, J. C/tern. Plays. 45, 2037 (1966).

b The value of xL20agrees with that obtained by J. C. D. Brand .TZ:~~ agrees with the infrared value obtained by Dr. W. J. Lafferty

S. J.,

et d. (to be published). The value of (private communication).