Volume 54, number 1 CHEMICAL PHYSICS LE-ITERS 15 February 1978

MULTIPHOTON TRANSITIONS 1N TRANS-BUTADIENE

OBSERVED BY MULTIPHOTON IONIZATFON AND THERMAL LENSING SPECTROSCOPY

Veronica VAIDA*, Robert E. TURNER **, John L. CASEY and Steven D. COLSON

Sterling Ckmistry Ldoratory, Yale LMkmity. New Haven, Connecticut 06.520. USA

Keccived 19 October 1977

The low-lymg two-photon states of trans-butadiene arc mvestigated using thermal lencing and multlphoton iomzation techniques. For the first time, the termal lensing method IS applied to the study of B gas-phase multlphoton spectrum. This spectrum is compared with those obtained by multiphoton romzation and liqurd-phase thermal Icnsin_r spectroscopy.

I_ Introduction

Recently, two techniques have been developed to study weak states in noncmitting molecules, namely multiphoton ionization (MPI) [ 1.21 and thermal lens- ing (TL) [3,4] spectroscopy. These techniques have also been shown applicable to investigating states ac- cessible by multiphoton absorption [ 1,2,4--61, adding a new dimension to the spectroscopy of molecules of high symmetry. In the cxpcrimcnts reported here, MPI and TL spectroscopy have been used to study the low- lying electronic states in trans-butadiene, a problem of considerable interest both from a theoretical and an ex- perimental point of view. Gas-phase multiphoton TL spectra are reported for the first time.

Early work on the absorption spectrum [7,8] of trans-butadicne was discussed by Mulliken [!I] in terms of g + u type transitions. More recently, cxpcrimcntal studies of ‘he low-lying states of higher polyenes [ 10, 111 have concluded that their lowest excited state is a g-state. The existence of low-lying excited g states in

smaller polyencs has been established by the two-pho- ton absorption spectrum of diphenylbutadicne [ 12]_

Extensive theoretical work has been concentrated on trans-butadiene, the smallest polyene [ 131. While the

* PrcFeet address: Department of Chemistry. Harvard Univer- sity, Cambridge, Massachusetts 02138, USA.

** Present address: Department of Chemistry, SUNY at Stony Brook, Stony Brook, New York 11794, USA.

second lAg state and a IB, state are predicted to be the lowest energy excited singlets, the relative posItIons of the 1 A,. and ’ B,, states vary amongst different calcula- tions. In r”eccnt optical studies [ 14,161 a vibronic hand has been identified on the highepergy tail of the low- est allowed singlet transition in trans-butadiemz and as- signed to the cxcitcd IA, state. More recently, the MPI spectrum of trans-butadienc [2] showed this to be a two- photon allowed transition. The vibronic analysis confirm- ed its reassignment [ 151 as a 1 Bg Rydberg state. The MPI

method detected no I A, excited state at lrlwer energies than the 1 Bg state, although its existence at higher en- crgics remained a possibility [2].

This study further investigates this low-lying ciec- tronic state of trans-butadiene. The MPl and TL tech-

niques available for such an investigation arc recent and still not well understood but the experiments rc- ported here allow them to be comparatively cvaluatcd. This is possible since we wcrc able to record the TL

spectrunl of gas-phase trans-butadiene. (This technique has been previously applied only to liquids in the UV- visible region [3,4] although an infrared TL effect has

been used to measure relaxation in gases [ 171.) The MPI and TL spectra of trans-butadiene arc essentially identical. The MPI spectrum of deuteratcd trans-buta- diene is also recorded for comparison. Iu order to in- vestigate the Rydbcrg versus valence character of the

two-photon state, the TL spectrum of liquid butadienc has also been studied.

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Volume 54. number 1 CHEMICAL PHYSICS LEI-KRS 15 February 1978

I 1 I _-.- 4000 3900 3800

Ldser wavelength (4)

Fi:ig. I. The hIPl spectrum of the two-photon aUowed state in perdeuteratcd tmns-butadlene. Tmcir.g (b) is a 3X eupnn- sion of (a).

2. Experimental results

The MPI spectra were obtained in the manner de- scribed by Johnson [1,2] and the differences in the ex- perimental apparatus used here were described previous- ly [5] _ The two-photon spectrum of trans-butadicne (C,H,) has been presented and discussed by .lohnson [2]. Here we present the corresponding MPI spectrum of dcuterated butadicne (C4Dg). The spectrum taken at a50 torr is shown in fig. 1 and the active vibrations are 8iven in table 1.

-ihe TL spectra were obtained using an apparatus that essentially consisted of a Molectron DL-200 dye laser (pumped by a nitrogen laser of our own design [ 181) as an excitation source used in conJunction with a He--Ne laser as the probe beam. The resultant, tran- sient TL signal and the energy of the linearly polarized

dye laser output (measured by a Molectron 53-05 pyro- electric joule-meter) were simultaneously integrated, recorded and normalized by a PDP-12 (D.E.C.) com- puter. All the spectra reported (both MPI and TL) were recorded by keeping the pulse energy of the dye laser constant during the scan as described by Johnson [ 1,2] _ Fig. 2 shows tile TL spectra of liquid and =2000 torr

vapor butadiene. The gas-phase TL spectrum just like the MPI spectrum [2] consists of a two-photon allow- ed transition with the origin at 3991 A superimposed on a weaker three-photon ionization continuum_ The TL spectrum of liquid butadienc (fig. 2a) consists of a strong absorption that be@ns around 4100 A and be-

comes more intense at higher energies. Trans-butadiene (C4Hs) was obtained from Mathcson and was purified by vacuum distillation. Deuteraled trans-butadicnc

(C4D6) was obtained from Mark, Sharp and Dohme, Ltd. and was vacuum distilled before USC.

--f----Y 440 --

Laser wavelength (nn~)

IGg. 2. The thcrm,d tensing spectrum of the two-photon al- lowed state in trans-butadienc. The gas-phase spectrum (a) is taken at about five times the sensitivity of the liquid (b). (a) Spectrum of butadicne vapor (=2000 torr). (b) Spectrum of liquid butadiene.

Table 1 Vibrational frequencies in the ground (I Ag) and two-photon excited sin&t of trans-butadiene (cm-‘)

____--_- -

Assignment Mode Butadicnehe Butadiene& -- ---- -- .-- - -.--

‘A&“) ‘Bzgb) lAga) 1 B,@ ---- --- _--- --_- ---

Da C=C stretch 1643 1593 1583 1546 Q5 C-II bend 1279 1248 1048 925 Da CH2 rock 890 907 739 705 Dg C-C=CIf2 513 469 440 409

a) Ref. [21]. b) Refs. [ 14.15]. C) This work.

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Volume 54. number 1 CHEhllCAL PHYSICS LETTERS 1.5 i-elmJary 1978

3. Discussion

The frequencies and assignments of the vibrations observed in the two-photon excited state for the C,H, and C,D, are given in table I. The active vibra- tions v4, vc, “8 and vg are all totally symmetric in ac- cordance with the two-photon allowed nature of the

observed state. In the one-photon VUV spectrum of C,H, and C,D,, the same state was observed via a vibronic mechanism [14,15] _ There, the active vibra-

tions wcrc vlO, v17 and v13 belonging to the A, block. The most intense vibration in the two-photon spcc-

trum is assigned as v4 and its very small shift upon deuteration agrees with this assignment. The 1248

cm -I vibration in C4H6 (assigned to either v6 or v7 19-I) has shifted to 927 cm- * in C4D6 and can now

be assigned as vs. a C--H mode. Table 1 gives a com- parison of the ground state and two-photon excited state frequencies for the protonated and deuterated trans-butadiene for all the active vibrational modes.

Next we will look at the line widths of the origins obtained in these spectra. One can rule out experimcn- tal artifacts both because variations in the relevant parameters (laser intensity, electric field strength in the cell and vapor pressure) did not charge the origin linc- width and by comparison of the MPI and TL spectra. The observed line broadening is probably due to a fast internal conversion process in that this level is below the lowest ionization or dissociation limits. While the protonatcd butadienc origin line is = 150 cm-’ wide (fwhm), the deuterated butadicne origin is reduced to = 100 cm-l_ Likewise, the broad lines in the lowest- energy one-photon allowed B, singlet spectrum be- come more narrow in the spectrum of the pcrdeuter- atcd molecule [ 14-161. These observations are con- sistent with an internal conversion broadening mccha- nism which is known to bc rcduccd upon deuteraticn through changes in the vibrational density-of-states weighted Franck-Condon factors.

In the spectrum of dcutcratcd butadiene, the rela- tive vibronic intensities have changed significantly when compared with the protonated molecule_ While this could result from isotope dynamic mixing [ 191, the additional change in the integrated intensity of all .vibrations with respect to the origin is not expected for an allowed transition. The origin contains most of the intensity in the deutcrated rnoleculc and only about half in the protonated species. At this point it should be

emphasized that intensities observed in a given MPI spec- trum may not be simply related to absorption cross sec- tions. In our case, the signal results from a two-step process, two-photon absorption followed by one-photon ionization. Both steps can affect the absolute or rela- tive cross sections. Furthermore, the dye-laser puise width can change with wavelength resulting in varia- tions of the peak intensity even though we maintain the

energy of each pulse at a constant level. This can have a significant effect on the apparent intensities ofmulti- photon transitions. Howevc r, the observation of the two- photon spectrum with the indcpcndcnt thermal lcnsing technique gives one more confidence in the MI’1 intcn- sitics.

The TL spectrum shown in fig. 2b and the corre- sponding MPI spectrum [2] give similar energies for the transition and, even more, the same mtcnsitlcs and line widths. The spectra are identical within the respec- tive experimental uncertainties. Understanding the in- tensity information obtained from multiphoton TL experiments is at an early stage. Here, the signal results from the heating of an absorbing sample by a focused laser beam. Because of the near-gaussian intensity pro- file of the beam, a tcmperaturc gradient is formed which in turn products a lens effect through the induced gradi- ent in the refractive index. The intensity of a second, low- energy probe beam (He-Ne) is monitored through a pm- hole after passing through the thermal lens. The transient reduction in the intensity of the probe beam is found to be linearly related to the strength of the absorption for one-photon excitations. However, w1w1~ one is monitoring two-photon absorptions, variations in the dye laser beam characteristics can affect the spectrum in muLh the same manner as they do in MPI spectra. Thus, while the near identity of the MPI and TL spectra may not be as telling as hoped, it IS nevertheless reassuring. Furthermore, the results demonstrate the usefulness of the TL technique for studying weak, UV-visible ab- sorption spectra of molecular gases. While the gas-phase spectrum of butadicne reported in fig. 2 was taken at x2000 torr we have found it to be detectable and cs- scntially wlthout change at pressures iis low as 100 torr [20], emphasizing the high sensitivity of the meth- od. The TL spectrum of liquid butadiene (fig. 2a) was also recorded in hopes of ftlrther clarifying the elec- tronic state assignment.

Because of its theoretical significance, further vcri- tication of the assignment of this transition would he

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Volume 54. number 1 CHEMICAL PHYSICS LETTERS 15 February 1978

useful in that it is based upon the identification of a weak hot-band found in the one-photon spectrum [14,15]. Since Rydberg states arc cxpccted to be very broad in the condensed phase, comparison between the gas-phase and liquid spectra should help assign the

states observed. This argument has been used [4] to reassign the states in benzene, by comparison of the gas-phase MPI and liquid-phase TL spectra and ruling out the possibility of observing Rydberg states in the liquid. Applying the same argument to butadiene, the signal seen in the liquid TL spectrum would be attri- buted to the Rydberg state, i.e. no sharp valcncc type spectral features are seen. This would then confirm the present assignment. However, our observation of the direct three-photon ionization continuum in the gas- phase TL spectrum calls this simple interpretation into question_ Note that the onset of the broad liquid spcc- trum occurs near the three-photon ionization threshold and, after raising to its maximum value, remains essen- tially constant. The shape of this curve is not affected by a two-fold reduction in the dye-laser power. This is what one would roughly expect for a signal due to ionization, suggesting an aitcmative expldnation for this spectrum and for similar broad features observed in liquid benzene [4] TL spectra. Since the thermal lensing signal is proportional to the net heating of thr: sample, the intensities observed in both the gas- and liquid-phase spectra will also reflect the effects of ion- ization and cannot necessarily be interpreted quanti- tatively by considering the two-photon Lross section alone [4]. The signal component due to the effects of ionization may in fact be large and mask any sharp features due to valence states. The liquid-phase continua have a quadratic dependence upon the light intensity, suggesting a two-photon assignment. However, because of possible saturation effects, the observed power de- pendence is frequently less than the order of a multi- photon transition. Thus, more work will be r1cede.i to establish the assignment of these broad liquid-phase absorptiond:

4. Summary

The recently developed techniques of MPI ‘and TL

spectroscopy have been applied to the investigation

of the two-photon allowed spectrum of trans-butadiene. The MPI spectrum of the perdeuterated molecule reveals

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isotope effects on the line widths and observed vi- bronic intensities.

The TL method is shown for the first time to be ap- plicable to the investigation of gas-phase multiphoton spectra. While the gas-phase MPI and TL spectra are composed of essentially identical sharp spectra on a continuous background, the iiquid-phase TL spectrum is completely continuous. A new interpretation involv- ing multiphoton ionization is suggested as a possibility for the broad spectra observed here and in other liquid- phase TL experiments. Because of this uncertainty, we are unable to make definite comment on the validity of the valence versus Rydberg assignment for the lowest ta

photon allowed transition in butadiene. It is hoped that high pressure (100 atm) TL spectra will provide this information through the known pressure effects on Rydberg transitions. Such experiments are now under way in this laboratory.

Acknowledgement

Financial support from the National Science Foun- dation is gratefully acknowledged. One of us (J.C.) would also like to thank the Camille and Henry Dreyfus Foundation for an undergraduate summer re- search stipend.

.

Note added in proof

The high-pressure TL spectra do in fact show pres-

sure broadening characteristic of transitions to Rydberg states, confirming the Rydbeg assignment for the lowest observedg-state in butadieno [20] _ Furthcr- more, at high laser intensities, we have observed large contributions to the “thermal iens” due to ionization. The resulting signal has different time and spectral be- havior than observed at the laser powers used in obtain- ing the spectra in fig. 2. Thus, it is more realistic to re- fer to this method in more general terms such as tran- sient lensing spectroscopy.

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Volume 54. number 1 CHEMICAL PHYSlCS LETTERS 15 t-ebrusry 1978

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