Relaxation of dihaloacetylene cations in their à and B̃ states studied by photoelectron-photon coincidence spectroscopy

Physics70 (1982) 32.5-328 North-Holland f’ubllshingCompany

Chemical

RELAXUIOPJ

OF DIHALOACETYLENE CATIONS IN THEIR x AND g STATES

STUDIED BY PHOTOELECTRON-PHOTON

COINCIDENCE SPECTROSCOPY

John P. MAI!ZR and Fritz THOM&lEN Phy~.~i-a~isch-C:le~nirches Insn’tut der UniversitiitBaseI, KiingelbPlgstrasse 80, CH4056 Bosei. SwitzerLmd

Received9 June 1982

Photoelectron-photon

coincidence

measurements

on the dihaloacetylene

cations, X-C=f-X’,

with X = Cl, Br, I and

Cl-CZC-BI+, in the gaseousphase are presented. True coincidences were detected not only following the formation of the cations in their x2nn,(g), a = 312,112, but also in their z’iI(,) states and the life&es and the fluorescence quanturn yields hwe_been determined. The decay curvesare exponen’kl, in contrast to other experiments, and whereas the lifetimes of the A states are in the IO-50

ns range those of the Estate

are in the microsecond

range. The relaxation

behav-

iour of these state selected cations is discussed. 1. Introduction

In connection with the study of the x 21J, + % ?H,, emission spectra of the dihaloacetylene cations, X-Cx-X*, X = Cl, Br, I, the decay cmves were also determined using e!ectron-beam excitation [I]. The deLzys were found to be non-exponential, &bough the long-lived component was two orders of magnitude weaker than the short. Later, photoion-photon coincidence measurements on Cl-@C-Cl+ showed a similar decay pattern [2]. By analogy with the pronounced nonexponential behavlour observed with the h&acetylene cations [3], for which the long component was only an order of magnitude weaker, the “intermediate case” model of radiationless transitions was used to rationalize these observations [2,4]. However, in contrast to the situation in the monohaloacetylene cations, fragmentation channels are not accessible from the R states of the_dihaIoacetyIene cations [S] _Thus population of the A state via radiative, or non-radiative, transitions from the Estate could not be entirely excluded, Photoelectron-photon coincidence spectroscopy provides an excellent means to probe the origin of the non-exponential decay because the internal energy of the cation is defmed [6]. The results of such measurements are cascade-free lifetimes and fluorescence quantum yields of the cations in selected vi0301-0104/82/0000-0000~~

02.75 0 1982 North-Holland

bronic levels. The application of this technique to the haloacetylene cations confirmed their nonexponential decay and allowed this to be investigated as a function of excess energy 171. The present measurements on the dihaloacetylene cations, X-C*-X+, X = Cl, Br, I and Cl-Cq--Br+, now clarify the situation concerning their relaxation behaiiiour in the x and B electronic states.

2.

Experimental

Photoelectron-photon coincidence curves were accumulated for the dihaloacetylene cations using the apparatus described elsewhere [S]_ Here only the basic features and principles are explained. The cations are formed under collision-free conditions (< 10-3 Torr) by photoionisation with He@) radiation. The internal energies of the cations are defined by the kinetic energy of the photoelectrons. The time intervals between the registration of energy selected photoelectrons and any undispersed photons originating from the ionisation regiion are then measured. The events are accumulated in a multi-channel analyser and the result is a curve of true coincidences superimposed on a uniform background of random coincidences. True coincidences arise only if the photoelectron and photon stem from the same cation of the specified

326

J.P. Hnier, F. Thommen/Relnration

energy. From the resulting coincidence curve the fluorescence quantum yield, &z(u’), of a seIected level u’ can be evaluated using the relation in&ma!

where I\‘~ is the rate cf true coincidences and Ne the rate of photoelectrons [S] . AbsoIute QF (u’) values can be determined because the system has been calibrated for the wavelength dependent transmission efficiency for detection of photons,&,@). The value #t: (c’) is then a mean of the fluorescence quantum yields of the rovibronic levels which lie within the band pass of the electronenergy malyser, typically SO meV ior 10 eV electrons. The Lifetime, rF (7~‘) is inferred from the decay cume by a weighted leastsquares fit.

3. Resdts

and discussion

of dfhaIoocet)lene

mtions

case of bromochloroacetylene cation (C,_,) are the 1: end g subscripts absent. All the coincidence curves measured for the Xi’-n R,g states were found to be exponential, in contrast to the electron impact data [l]_ The inset of fg I shows one such curve for dibromoacetylene cation in the lowest level of the x2ii3,2,p state. The determined lifetimes and the corresponding fluorescence quantum yields aE summarized in table 1. The given lifetimes for the A states agree with the values of the short component of the quasi-biexponential decay curves which had been found in the emission experiments using a puked electron beam [I ,9] _ Thus the longcomponent

decay is an artefact of the non-specificity of the eiectron-impact excitation. The photoion-photon coincidence studies on dichloroacetylene cation have, however, shown directly that the photons which contribute to the long component of the quasi-biexponential decay CUIV~stem from the parent ions and not from fragments

Photoelectron-photon coincidence measurements were carried out at the ionisation energies given in table 1. As an example the He(la) photoelectron spectrum of dibromoacetylene recorded under the coincidence conditions

is shown in fig. 1 and the selected

energies are indicated

by the arrows, For the “A%,

g,

R = 3jZ, l/2 states, this corresponds to the lowest vibrational ievek As the whole of *he following discus-

[2]

_

This was aiso concluded from comparison of the ‘timeresolved emission spectra, an exarrple of which is shown in fig. 2. The emitted photons Jie in the same ener,y region for the short and long components. Furthermore, bands were not observed in the wavelength re@on corresponding to the dipole forbidden -7 B -fI, + X 2% transition and the allowed transition,

sion is focused primarily on the symmetrical species, the D _,h symmetry classification will be used in generai reference to the “dihaloacetylenes” as only in the

T !

I;$. I. II-:

photoelectron

spectrum

.

of dibromoacetylene

shoivir,: the levds studied. The imets shots the photoelectronphoton cci+ience curves accumulated 3: the indicated positions for the .4’r’rgn, s (left), and E’?r& (right) states.

F&. 2. Time-resolved emission spectra of CLC-S-C!’ recorded for the indicated time intervals, with 1.5 nm resolution and excited by ~30 eV electrons (from ref. [ 1I).

J.F’. Maw, F. Thommet&ehztion

g211,, -+ x 211g, lies outside the detection region of the photomultipLier (x > 900 mn) [l] . The origin of the long component is revealed by the photoelectron-photon coincidence measurements on the dihaloacetylene cations in the K ?lIu state (see inset fig. 1). True coincidences were detected and lifetimes inferred from the decay curve are given in table I They are comparable with the values of the long component from the emission experiments, some of which are included in table 1. The given lifetimes and fluerescence quantum yields represent only the lower hrnits because of the restricted observation region for such long-lived ions. Fir&y, using wavelength selective filters, it was confiied that the photons which are emitted following the initial preparation of the cations in the iii*II, state lie in the energy region of the x ‘Ii, + !? zII, transitions. In view of these observation: the decay behaviour of ‘he dihaloacetylene cations produced in t&ir x 2n and -7 B -II, states can g be discussed.

As the fragmentation thresholds for all the dihaloacetylene cations he energetically well above the populated part of the E’II, state [S], this decay channel is not accessible. The possible decay processes for the

321

of dihaloaceryiene cations

cations in their x 211g states are the radiative relaxaand the internal conversion tion, A 211 + “x zlI x 211r-+!%* 2&r .%e lowest lying quartet stat: is expected to he to higher energy, at similar linear geometry. The vibrationally excited levels of the groilnd state, 2, form a quasicontinuum for the initiaIly populated isoenergetic levels within the x state. Monoexponential decay curves and less than unity fluorescence quantum yie!ds are typical of species in the statistical limit [lo]. The determined fluorescence quantum yields and lifetimes for the x 211l/z &)O” level of Br-C%--BP (and Cl-CS-Br+) are s’kaller than for the Xi’II ;,2,kIOo level. A possible explanation for this is that the cations produced in the !El= l/2 state are coupled not o_nly to the isoenergetic levels of the ground state X but also to the levels of the x 2II3,2 state. In the case of Cl-Cx-Ci+ the two spin-orbit components could not be separated with the attainable resolution, whereas for I-(EC-Ii the evahrated fluorescence quantum yields (see table 1) are uncertain. This is because ‘the quantum effkiency of the photon detection system strongly decreases above 750 run and therefore the value for the 0, = 3/Z state

Table 1 Fluorescence qumrrrm yields, +(v’), and Iihtimes, rF(u’), of the dihaloacetylene The values beg are evaluated from the biexponential decay curves from refs. [ 1,9]

cations

measured at se&ted

Ion

state

IE (A’)

OF (v’)

Cl-CkC-cl+

X2n nL,g

13.37

0.38 f 0.04

13 * 2

13.47

0.36 f 0.04

13 f 2

E’n,

Cl-C=C-Bf

14.40

i2.54

0.39 t 0.04

12.73

0.09 k 0.02

14.10 Br-GC-Br+

a) Lower limit, see text.

=o.u

a)

-2s50

a)

21 i 2

=lioo

a)

12.12

0.99 i 0.05

31 i 3

0.67 k 0.05

2.5* 3

~0.05 a)

10.63

=o.ii

11.24

==0.61 a)

a)

12.40

~0.04 a)

13 5 2 13 + 2 =I100

23 i 2

~700 29 f 3 27 + 3

a)

~850

25 f 3 a)

-41

-1500

52i3a) >3000 a)

enerpies.

Tem(ns)


12.42 13.40 I-c=C-I’

==0.13 a)

rF(u’)(“s)

intemel

51 f 3 >4000

may be too small. However, the lifetimes obtained from the coincidence and electron impact experiments are also shorter for the n = 312 component. Thus the coupling of the ir@ially prepared Q = l/2 state to the G = 312 component of the A zl?,g state appears to be attenuated due to the larger energy gap between these states (eO.6 eV) than in the other dihaloacetylene cations (cf. table 1). The observation +&at the lifetimes for the g states are larger than for the “Astates is unusual for openshell organic cations. For species such as the halo substituted benzene cations where the bigher excited states are also depleted radiatively via the fi;st excited state [ 111, ~~ (s) > TVt$) and reflects the irreversible radiationless transition z -+ 2 _In these cases the density of isoenergetic levels of The x state is around 1 Oio/cm-l. On the other hmd the corresponding density of states for Ihe dihaloacetylene cations is about lOj/cm-1 and thus the r&x&ion behaviour of their g states can be explained with an intermediate level structure Q]. Isoenergetic levels of the A state provide both a quasi-contizuum and discrete levels for the initially populated B state Ievel. The resultant resonant states with g and x character carry oscillator strength to the cationic ground state X. Thus the J3 character is not irreversibly lost, leading to long lifetimes. The small fluorescence quantum yields given in the table, which may be sonewhat too low, reflect-the irreversible radiationless rransitions from the B state to the quasicontinuum formed by the x and ? states. As a consequence of the discussed relaxation behavizur of$e dihaloacetyiene cations prepared in the A and B states, the time resoIved emission-spectra shown in fig. 2 have different origins. The spectrum of the short component corresponds to the radiative transition Z-+ Z, while the one of the long compo-

nent represents the result of ‘&e radiationless coup.li$ z --+ 2 followed by the radiative transition ;i* --f X* between the highly excited vibronic levels of the two states.

Acknowledgement This work is part of project no. 2.017081 of the Schweizericher Nationalfonds zur Fdrderung der wissensch&lichen Forschung. Ciba-Geigy SA, Sandoz 5% and F. Hoffmann-La Roche & Cie., Base1 are also thanked for financial support.

References

[ 1j 51..4Uan,

E. Kloster-Jensen and J.P. Xi&r, J. Chem. Sot. Faraday II 73 (1977) 1417. [ 21 G. Dujardin, S. Leach, G. T&b, J.P. hlaier and WM. Gelbart, J. Chem. Phys. 73 (1980) 4987. [ 31 hf. Alian, E. Kloster-Jensen and J.P. Bfaier, J. Chem. Sot. Faraday iI 73 (1977) 1306. [4] S. Leach, G. Dujardin and G. Taieb, J. Chim. Phys. 77 (1980) 705. [5] H. Baumgtitel, E. Kloster-Jensen, IV. Lohr, J.P. Maier, H. ijrtei and H. She& unpublished work, v&es quoted in refs. (1,3]. [6 ] bf. Bioch and D.W. Turner, Chem. Phys. Letters 30 (1975) 344. [7] J.P. Maier and F. Thommen, in: Intramolecular dynamics, eds. .I. Jortner and B. Pullman (Reid& Dordrecht, 1982). IX] 1-P. Maier and F. Thornmen, Chem. Phys. 51 (1980) 319. [9] W. Wan, 0. Marthaler and J.P. M&r, unpabfished results. [lo] K.F. Freed, Topics Appl. Phys. 15 (1976) 23; P. Avouris, W.M. Gelbart and X.4. El-Sayed, Chem. Rev. 77 (1977) 793. [llj J.P. Maierand F. Thommen, Chem. Phys. 57 (1981) 319; J.P. Maier and F. Thommen, J. Chem. Phys. 77 (1982), to be published.