Radiative and predissociative decay of electronically excited alkali hydrides

Journalof Molecular Structure, 143 (1986) 565-566 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

RADIATIVE

AND PREDISSOCIATIVE

DECAY OF ELECTRONICALLY

565

EXCITED

ALKALI

HYDRIDES

H.H. TELLE Department Swansea

of Physics,

SA2 SPP

University

College

of Swansea

(United Kingdom)

ABSTRACT

Radiatiye lifetimes for all vibrational levels of the first excited singlet state A 11 of the alkali hydrides NaH and CsH have been calculated. The results are compared with scxne experimental values, and the role of predissociation is discussed,

INTRODUCTION The lowest electronic ple for ionic-covalent crossing

between

harmonic

ground

depth)

states of the alkali hydrides

interaction

the M+H and M++Hstate potential

and an extremely

sitions

between

probing

radiative

transition

potentials

for the excited

The related

an order of magnitude

We have calculated

with its absolute intensity

ratios

example,

emission

intensities

by the calculation

for weaker Franck-Condon

as much as an order of magnitude. dipole moment agreement

function

information

and lifetimes

func-

separation

excited

state.

large ensem-

the measurement

for all vibrational

for many transitions

the numerical

of

(ref. 2, 3).

in the A-X band are

modification

intensities

(ref. 4).

0 19236 Elsevier Science Publishers B.V.

For

trend, but especially

fit procedure

emission

levels

with our measure

values occasionally

Only an appropriate

and calculated

bars for most of the transitions

moment

on the transition

are canpared

data for NaH

u (A-X) in a least-squares

between measured

QO22-2860/86/$03.50

transition

with internuclear

at least in their general

transitions

Tran-

magnitude.

(ref. 1) and sane experimental

relative

reproduced

well

state A lC+.

levels in the electronically

then allows to obtain

in the A 'I+ state of NaH and CsH, and the results for CsH

gives rise to a nearly

bands reveal the shape of this function,

is connected

exam-

of the potential

; while relative intensity ratios of a sufficiently

ble of vibrational

ments

a unique

The avoided

band systems which are sensitively

probabilities.

high vibrational

A series of measurements

lifetimes

potential

these two states provide

when investigating

function

interaction

represent

behaviour.

(for more than one-third

anharmonic

tion n(A-X) varies by almost

moment

and non-adiabatic

differ by

of the

finally brings within

the error

566

Fig. 1.

Molecular data for the X-states of NaH; _1 (a) potential curves (in cm ), the dashed curves indicate the diabatic representation; (b) dipole moment function (in a.u.) from ref. 5; (c) radial coupling
RESULTS For the calculation lated intensities necessary,

the knowledge

together

intermediate

of the wave functions of the upper

with transition

range of internuclear

data, whereas

for distances

moment

The potentials

separation

distances

are based on ab initio calculations

moment

function

A-coefficients. excited

state

observed

vibrational

have been introduced

for NaH is displayed Eigen

functions

to calculate

represented

is

are in the

by experimental

RXR

are taken; for procedures

for NaH are shown in Fig. la, and the RXR range is

by the highest

The calculated

The potentials

the ccmmon extrapolation

functions

fied function

and the re-

and lower state potentials

function.

indicated

CsH), and modifications

mcments

up to 10.5 E( scaled ab initio values

very small and large internuclear were applied.

and transition

The transition

moment

(see ref. 5 for NaH and ref. 6 for as outlined

in ref. 4; this modi-

in Fig. lb. Iv,J> were used in conjunction

the transition

The summation

levels.

matrix

of the Einstein

1x1',J'> over all vibrational

elements

A-coefficients

with the dipole

and the Einstein for a specific

leyels of the ground

state and the

567

Fig. 2. Lifetimes for the A 'I+ state (in nsec), as a function of vibrational quantum number v'; full symbols stand for calculated values, open symbols for experimental data. (a) CsH for J'=ll and (b) NaH for J'=b.

(8) -

!4s-

4.0

1

s(Dlowwww V’

P- and R-branches that because

result

in the radiative

of the extreme

anharmonicity

sion from very high vibrational energy

lifetime.

transition

levels is found to be continuum

transitions

probability.

out

in the A 'Z+ state most of the emis-

levels above the X lZ+ dissociation

radiative

It should be pointed

limit, comprising

Contributions

to the B 'If state and IR transitions

emission

up to 85% of the

to the emission within

to

intensity

by

the A IC+ state are also

taken into account. The calculated values

ai

lifetimes

levels,

and then drop rapidely

for the dissociation trend is generally where a decrease

product

levels for CsH. thought

to finally

reach the value

with M being the alkali atom.

well reproduced;

however,

a rapid increase

already

This rapide decrease

for moderately

For NaH this

of r is measured

On the other hand, the lifetime

values

with experimental

vary only weakly with v for low vibration-

as v increases,

M('P+)

is expected.

cantly below the calculated

in Fig. 2, together

are representd

The lifetimes

(see ref. l-3).

drops signifi-

high vibrational

to an approximately

constant value

for the MH molecule

are indicated

is

to be due to predissociation.

Diabatic

potential

representations

la by the dashed

lines.

coupling

in a certain probability

results

state can predissociate elaborate scattering

In a simple Landau-Zener

matrix

diabatic/adiabatic

that a vibrational

along the diabatic

Landau-Zener-Stuckelberg

picture

approach

(see ref. 5); the radial

ref. 8, and it is shown for NaH in Fig. lc.

curve.

level in the A 'L'+

We have chosen the more

in the two-state coupling

in Fig.

function

approximation is taken from

of a

568 The calculated

predissociation

only for a few percent to explain

the significant

dissociative turbative

rates for high vibrational

drop in lifetimes

channel may be responsible

coupling between

can be deduced

contributions

observed

and a possible

only from extrapolation.

effect.

crossing

A second preThis is per-

are presently

with the A state

Some calculations

to the decay rate of a given vibrational

tive predissociation

for CsH.

for the observed

large

the A 'C+ and a 3Z+ states; the latter is only

known from ab initio calculations, potential

levels account

of the total decay rate and are not sufficiently

to include

level through perturba-

under way.

CONCLUSION We have shown that theoretical levels in the first electronically reproduce

experimental

dissociative investigation

channels

findings

calculations excited

reasonably

well.

seems to be necessary,

at present

of the lifetimes

for vibrational

state, A 'Z+, of the alkali hydrides

in this laboratory,

However,

the inclusion

and their contribution and lifetimes

of pre

is under

for very high vibra-

tional levels are being measured.

REFERENCES 1 M. Ferray, J.P. ViStiCOt, B. Sayer, H.H. Telle, J. Chem. Phys., 81 (1984) 191-194. 2 P.J. Dagdigian, J. Chem. Phys., 64 (1976) 2609-2615. 3 0. Nedelec and M. Giroud, J. Chem. Phys., 79 (1983) 2121-2125. 4 H.H. Telle, J. Chem. Phys., 81 (1984) 195-201. 5 E.S. Hinse, J. Hinse and N.H. Sabelli, J. Chem. Phys., 62 (1975) 3384-3388. 6 B. Laskowski and J.R. Stallcop, J. Chem. Phys., 74 (1981) 4883-4887. 7 H. Nakamura, J. Phys. Chem., 88 (1984) 4812-4823. 8 (a) L.R. Eguiagaray, L.F. Errea, L. Mendez, 0. MO and A. Riera, in W.J. Merz and G. Thomas (Eds.), Europhysics Conference Abstracts vol. 98, EPS, Amsterdam, 1985, p. 326; (b) 0. MO, private communication.