Assignment of the E absorption transitions in C6D6 by means of isolated-molecule fluorescence spectra

CHEhllCAL

Volume 99, number 5,6

ASSIGNMENT

OF THE E ABSORPTION

BY MEANS OF ISOLATED-MOLECULE

19 August 1983

PHYSICS LETTERS

TRANSITIONS FLUORESCENCE

IN C6D6 SPECTRA

M. SUMITANI, D. O’CONNOR*, N. NAKASHIMA, K. KAMOGAWA, Y. UDAGAWA and K. YOSHIHAFU

Y. TAKAGI,

instifure for Molecuh

Science, n-lyodaiji, Okazaki 444. Japan

Received 26 hlay 1983

Pluorescence spectra following narrow bandwidth excitation of CeDe have been observed and their structure analyscd. Excitation of tbe Et absorption band gives rise to fluorescence that cannot originate from a level with r,a excited.

ture in accordance

l_ Introduction

While the fluorescence spectroscopy of benzene under isolated-molecule conditions has been well studied [ 11. fully deuterated benzene has received little attention. We are aware of only one reported emission spectrum [a], that of the zero-point level, from C6D6 vapour under collision-free conditions. Yet as Abramson et al. [3] have shown, the study of the photophysical behaviour of S, C6D6 as a function of excess vibrational energy provides an informative parallel with the study of C6H6 _Therefore as part of an investigation of channel 3 decay in benzene [4,5] we have measured fluorescence spectra and decay times of C6D6 in an attempt to isolate the role of C-H vibrations in the nonradiative decay. Since our interest is in channel 3 behaviour we have excited transitions lying close to or above the threshold of this process. Compared to the absorption spectrum of C6H6, that of C6D6 is severely congested, so much so that, at room temperature, it is impossible to populate single vibronic levels cleanly. For this reason all our observed spectra, although measured with a narrow band laser excitation source and under low-pressure conditions, have contributions from several individual levels. Nevertheless for many spectra it is possible to assign the struc-

* Present address: Tlre Royal Institution, 21 Albemarle Street, London WlX 4BS, UK.

0 009~2614/S3/0000-0000/s

03.00 0 1983 North-Holland

with the principles

of vibronic

ac-

tivity governing the absorption and fluorescence spectroscopy of C&H6 [ 1,6] _in a recent publication [6] we have shown that excitation of a number of transitions in C6H, lying at wavelengths to the blue of the channel 3 threshold leads to fluorescence spectra that can be attributed, at least in part, to the level that is optically prepared_ In C6D6, on the other hand, when the excitation wavelength is less than ~240 nm almost all recognizable vibrational structure is absent from the spectra. It is highly likely that the reason for this lies in sequence congestion since at these wavelengths the absorption spectrum is also rendered relatively featureless by overlapping transitions and hot band excitation. Of the four e?, vibrations in benzene it is now known [7] that only ti:, u6 and v7, play a major role in in-

ducing the absorption transition_ In the first detailed analyses of the C6D6 absorption spectrum [S], Sponer tentatively assigned a progression, which she labelled with the letter E, to a one-quantum excitation of a tlrrrd e2g vibration “g_ This assignment was also given [9] for the E progression in C6D6 and was followed for both molecules in the more comprehensive analyses of Garforth and Ingold [IO]. It was, however, later demonstrated by Callomon et al. [ 1 l] that the assignment was probably incorrect. These authors attributed the E series in C6H6, and by analogy, in C6D6, to a progression in the totally symmetric ring-breathing vibration ni built on a one-quantum excitation in “6 in com445

Volume 99, number 5,6

CHEMICAL PtrYSICS LETTERS

bination with a two-quantum excitation in the out-ofplane elg vibration vlo_ In spite of this authoritative analysis the presence of absorption bands induced by “8 has continued to be a matter of controversy [ 12,131. Quite recently the E bands in C6D6 were again tentatively assigned to a one-quantum change in v8 [ 141. Moreover it has been demontmted by Pamlenter and co-workers. by analysis of the absorption specrrum [ lS] and SW_ emission spectra [I], that the E bands in C6H6 arise from excitation of one quantum of p6 in combination with one quantum each of the out-of-plane ezu vibrations VI6 and vt7. We show in this paper that in C6D6 the assignment of Callomon et al. [ II] is probably correct_ The almost exact analogy of this band series in C6H6 and C6D6 is therefore coincidental.

t 310

19 August 1983

1

I

1

f

,

1

300

290

280

270

260

250

1

2f.O

I

230

Wavelength /nm Fig. 1. Fluorescence spectrum following excitation of the Ei absorption band in CsD6 at 736.3 nm. The scattered light si_end at this wavelength is not drawn in fully.Sample pressure29 Torr. Fluorescence collected in a 5 ns gate width centered 3.5 ns after excitation.

t. Experimental C6D6 with a stated isotopic purity of 99.99% was purchased from Stohler Isotope Chemicals. it was purified in the manner already described for C6H6 [6]. Emission spectra were usually measured with the sample pressure at 90 mTorr but for short-wavelength excitation the srirnpIe pressure was increased and the spectra were gdted at early times 161, Full details of the escitation source and optical multicham~el analyser detection equipment have been given elsewhere [6.15] _Of relevance to the present paper are the spectral band~vidth of the exciting light, which was 10 cm-l, and the spectral resolution. which wds MO CIII-~. In denoting absorption and fluorescence transitions and vibrational levels in So and S, we use the wellknown notation of Callomon et al. [ 1 I]_ We also use the older notation EF which was a label given to transitions in which one quantum of v8 and II quanta of ark are excited in St, with NI quanta of u1 excited in So_ Vibrational energies were calculated in accordance with the tabulation of nornlrtl mode frequencies given by Abramson et al. [3]_ Absorption assignments are taken. for the most part, fro:n the compilation of Carforth and lngold [IO].

3. Results The electronic

lies at 38290 cm-t 446

origin of the S,(tB& state in CsD6 IlO] _Since the ~vaveIengtI~s avail-

able with our excitation source lie in the range 251-227 nm we could populate levels having excess energies of roughly 1550-5750 cm-l. Excitation of almost all the absorption bands below 240 nm led to structureless fluorescence spectra. For example, following excitation of the relativeIy strong E$ absorption band at 2362.6 a the spectrum illustrated in fig. 1 was observed_ No assignable features can be discerned in this spectrum (which was measured at high sample pressure, emission from levels populated by collisional relaxation being excluded by gating the data collection), whereas spectra from C6H6 at all wavelengths of excitation have clear vibrational features [6]. Given the congestion present in the C6D6 room-temperature absorption spectrum at this wavelength it would be unwise to draw any conclusions about the nature of channel 3 from the lack of structure in the fluorescence spectrum_ Unfortunately the same conclusion must be drawn with respect to all excitation wavelengths below the channel 3 threshold and for this reason we confine ourselves in what follows to a discussion of the structured emission resulting from longer-wavelength excitation_ Virtually all the bands we have excited at longer wavelengths are bands having some contribution from E transitions. An exception is the absorption at 2448 a which results from the transition 6: 17: I& The fluorescence spectrum resulting from this excitation is shown in fig. 2. As the assignments show the structure

is compIeteIy compatible with fluorescence from the

Wavelergth no

1

300 I

290 1

280 1

19 August 1983

CHEMICAL PHYSICS LETTERS

Volume 99, number 5.6

Wavelergth / nm

/ nm

270 I

260 I

250 I

2.50 I

230 1

Fig. 2. Fluorescence spectrum resulting from excitation of the 6;17; 1: transition at 244.8 nm. The height of the vertical bars on the line representing the 62 17s 1, fluorescence progression correspond to the expected intensity for the members of every 1 1 -+ 1, progression as calculated in ref. [6]. The scattered light signal at the excitation wavelength has not been drawn in fully. Sample pressure 90 mTorr. Ungated spectrum.

level 6ll 72 1 1 while the broad background can be attributed to congested emission resulting from hot band excitation as well as to the rather low resolution of our measurement.

Having shown that, at longer wavelengths, fluorescence spectra characteristic of the level excited and hence of the absorption transition can be observed we now turn our attention to the strongest band in this region centered at -2455 A_ When we measured the fluorescence excitation spectrum of this band, scanning the excitation wavelength in I A intervals, only a single peak with a shoulder at shorter wavelength was observed_ The shoulder is due to the 7: transition and the peak is composed of the two overlapping transitions, 66 1; and Ey , centered at 40546 cm-l (246.6 run) and 40573 cm-l (246.5 nm), respectively [IO] _ In fig. 3 is shown the emission spectrum resuiting from excitation at 246.6 nm. It is characterized by two main progressions, one of which can be assigned to fluorescence from 6’ l?- since it possesses the intensity distribution expected from a level with u; = 2 [6]. This distribution has a minimum at u’; = 3. (It should be remarked that the peak labelled a in fig. 3, at a displacement of 3780 cm-I from the excitation energy, does

310 I

300 I

290 1

200 I

270 I

260 I

250 I

240 I

Fig_ 3. Fluorescence spectrum resulting from exitation of the Ey transition at 246.6 nm. The signal at this wavelength is mostly scattered light but may have some contribution from resonance fluorescence. Peaks a and b, separation 909 cm-‘, have not been assigned. Sample pressure 90 mTorr. Ungated spectrum.

not correspond to the transition 61 lz, which would have a displacement of 4000 cm-I and which seems to be absent from the spectrum.) This very low intensity is in agreement with a simplified overlap calculation [6] which indicates that the intensity ratio of 6: i$ to 6: 1: is ~50 : 1. The other progression, labelled X, is not due to 6’ l2 fluorescence since its origin at 263 nm (displacement 2453 cm-l) is the most intense peak in the spectrum whereas the most intense transition from 61 l2 would be 6: 1i at 253 nm. Nor is it due to 7’ fluorescence, the most intense band of which, 6:7:, would lie at a displacement of 2854 cm-l from excitation. It must therefore have its origin in the level populated by the E(: absorption transition. Knight et al. [l] have listed the possible candidates for the Eg transition in CeHe. It is generally assumed that E: is the band origin of the En progression so that the plausible assignments for Eg can be reasonably applied to E(: if excitation of a further quantum of “I is included. In table 1 we have grouped the possible assignments for Ey derived in this way from the list of Knight et al. Also included in table 1 is the transition 6; 10; 1 h proposed by CaRomon et al. [ 1 l] _ In table 1 absorption energies in C6D6 as well as the energy of the first

member of the most likely strongest fluorescence pro447

CHEhlICAL

Volume 99. number 5.6

19 Au-at

PHYSICS LETTERS

1983

Table 1 Possible assignments for the E’: transition in CeD 6_ In the last row of the table are shown data for the observed E’: transition as well as for the fluorescence progression labelled X in fig_ 3 Transition

S;l;

40572 (246.5)

Strongest fluorescence progression notation

disphcement of ori& (cm-‘)

intensity distribution

6’5’ 11n 1’

2137

1’ -

I,,

a)

6:11;1:,

4035’ (247.8)

6$11;1,:

2150

l’-

1,

6;16;1:,

40500 (146.9)

6:16:1,:

2538

1’ -

I,*

6;16;17&1:,

40466 (247.1)

6:16:17:1;,

2290

1’ -

&I

6;3;1:,

40606 (246.3)

6;3:

1,:

2217

l’-rl,

6;4;10;1;

40418 (247.4)

6;4;

IO; 1;

2416

l’-+l,

6;10’01;

40576 (246.4)

6;10;1,:

2476

1’ -

Ey

40573 (246.4)

I1

2453

1’ + 1,

a) Values in pxenthescs

gression

Absorption ener,T (cm-‘)

are given.

in nm.

of absorption energies 6; 11; I A, 6; 16; 17; 1; and 6,$4,$10; 1A. Given that we can measure fluorescence displacements to an accuracy of =lOO cm-l we must also rule out SA 16 and 6;3; 1;. There remain 6; 16401A and 6h 10; 1A_The former is expected to be a weak transition [ 1] and would not give rise to the strong intensity observed in progression X. We therefore conclude, as 11~sbeen tentatively suggested previously [ 1 I], that the E band series in C,D, can be assigned to the progression 6; 10; 1II. o in contrast to C6H, where it is delinitely established [ 1] to 6: 16: 17,$ I;_ alone

On the basis

we c3n rule out

4. Conclusion Owing to its congested absorption spectrum C6D6 does not lend itself as well as C,H6 to investigation by single vibronic level spectroscopy. In particular there appears to be little information to be gleaned about 44s

I,,

channel 3 from spectra measured following room temperature absorption. Below a certain excess energy, which we feel is purely by coincidence close to the channel 3 threshold, however, structured spectra can be observed and analysed in accordance with the general pattern of vibrational activity observed in the 1 B2,,--I Alg transition in benzene_ As a result we are able to conclude that the Ey absorption band in C6D6 is not induced by the vg mode but, in common with most of the absorption in C6H, and CsD6, by v6_ It may be worthy of note, however, that we are unable to assign two relatively strong peaks labelled a and b in fig. 3, in the fluorescence spectrum that results from E, absorption_ These peaks increase in relative intensity as the excitation wavelength is decreased slightly but no completely satisfactory assignment can be proposed. It is remarkable that similarly strong unassignable bands were observed in the fluorescence spectrum of the level 611021t inC 6 H 6 _

Volume 99, number 5,6

CHEMICAtPHYSICS_LETTERS.-

References [I 1 A-E-W. Knight, C-S. Parmenter and M.W. Schuyler, J. Am. Chem. Sot. 97 (1976) 1993.2005, f21 C-S. Pannenter and M-W. Schuyler, J. Chem. Phys. 52 (1970) 5366. j3 3 AS. Abramson. K-G. Spears and S-4.. Rice, J. Chem. Phys. 56 (1972) 2291. [4 J Y. Takagi, &I. Sumitani, D. O’Connor, N. Nakashima and K. Yoshiham. J. Chem. Phys. 77 (1982) 6337. [5] M. Snmitani, Y. Takagi, D- o%onnor, N. Nakashima, K, Kamogawa. Y. Udagawa and K. Yoshihara, Chem. Phys_ Letters 97 (1983) 508. N. Nakashia, IL [ 61 D. O’Connor. M. Sumitani, Y _Tak3gi, Kamogawa, Y. Udagawa and K. Yoshiham, J. Phys. Chem., submitted for publication.

19 August 1983

171 G-H. Atkinson and CS. farmenter, 5. Mol. Spectry, 73 (i978) 20,3J, 52. fSJ H. Spouer, J. Chem.Phys. 8 (1940) 705. [91 H. Sponer, G. Nordheim, A.L. Skhx and E. Teller, J, Chem. Phys. 7 (1939) 203. [IO] F.M. Gartorth and C-K- ingold, J. Chem. Sot (194s) 4 $7, 433. [ 111 J.H. Callomon, TM. Dunn and I-hi: hlills, PhiL Trans. Roy. Sot, (London) A259 (1966) 499. [ 12) F. Met& I&3. Robey, E.W. Schlag and F. Diirr, Chem. Phys. Letters 51 (1977) 8. [ 131 G. Fischer, Chem. Phys. Letters 56 j1978) 1%. [ 141 G. Fischer and S. Jakobson, Mol. Phys. 38 (1979)299. 1151Y. Takagi, hi. Sumitani, N. Nakashima, 1). WConnor and K. Yoshihara, Appl. Phys. Letters 42 (1983) 486.

449