- Email: [email protected]
Low energy, variable angle electron-impact excitation of 1,3,5-hexatriene
CHEMICAL PHYS!CS LEITERS
Volume 45. number 3
1 February
1977
LOW ENERGY, VARIABLE ANGLE ELECTRON-IMPACT EXCITATION OF 1,3,5-HEXATRIENE* Wayne
M.
FLICKER, Oren A. MOSHER$ and Aron KUPPERMANN
Arthur,Amos Noyes Laboratory o/Chemical Pasadena. California 91125. USA Received 3 November
Physics*.
California Institute of Technology,
1976
A mixture of cis and tram 1,3,5-hcxatricne has been studied by electron impact at incident electron energies of 20 cV, 40 eV, 50 eV. and 70 eV, at scattering angles from 0” to 80”, and with effective energy resolutions in the range from 0.05 eV to 0.15 PV. Singlet -L triplet transitions with maximum intensities at 2.61 eV and 4.11 eV are observed. The lowest cncrgy spin-allowed excitation which can be detected is rhc electric dipole-allowed % *A g + 1 ’ Bu transirion (in the notation appropriate for the trans isomer). No evidence has been found for a spin-allowed but symmetry-forbidden z *Ag + 2 *Ag excitation in the vicinity of 4.4 eV transition energy. Many other spin-allowed excitations are observed in the 6- 11 eV energy-loss region, and the correlation between these features and those observed in high resolution ultraviolet absorption spectra and other electron-impact spectra is discussed.
In the past five years, there has been a marked renewal of interest in the electronic spectroscopy of linear polyenes [l-l 81. Of practical concern is the fact that many molecules with important photobiological functions, such as Vitamin A, Vitamin D precursors, and retinal and other carotenes, contain polyene chromophores involving conjugated double bonds. The photophysical pathways by which the absorbed light energy is degraded in these molecules are largely unknown [ 19,20], prim:lrily because until recently there was relatively little information available about the electronic spectroscopy of such molecules. We have previously reported the results of extensive electron-impact investigations of the prototype conjugated polyene, !,3butadiene i4,16], and in this paper, we detail a similar study of 1,3,5hexatriene. Both threshold [6] and high energy [ 1 l] electronimpact investigations of hexatriene have been reported recently, but the limited resolution of the spectra in * Work supported in part by the U.S. Energy Research and Development Administration, Contract No. E(04-3)-767. Report Code: CALT-767P4-149. * Work performed in partial fulftient of the requirements for the Ph.D. degree in Chemistry at the California Institute of Technology. 3 Coniribution No. 5461.
both of these experiments has made it difficult to compare the results quantitatively with high resolution ultraviolet absorption spectra [5,8 1. In the present work, we have obtained variable angle electron-impact spectra at higher resolution than previously reported [6,1 I]. This permits us to clarify the nature of the weak absorptions observed at 4.2 eV [6] and 4.4 eV [ 111. In the conduct of this work, we were most interested in determining whether the lowest energy singlet + singlet transition in hexatriene is electric dipole-allowed, as most experimental data indicate [5-81, or whether it is electric dipole-forbidden, as recent theoretical calculations predict [2,6,8,12,14]. The spectrometer employed in this investigation has been described in detail previously [21,22] _The experiment consists of directing a collimated, energyselected electron beam onto a target gas, contained wzthin a flexible-bellows collision chamber. The kinetic energy distribution of the scattered electrons, at a preselected scattering angle, is then determined by sweeping repeatedly the energy-loss range of interest. The measured electron energy distribution, or energy-loss spectrum, corresponds to a photon absorption spectrum, except that it frequently contains appreciable contributions from both spin-forbidden and spinallowed but electric dipole-forbidden processes, as well
Volume45, number3
CHEMICAL PHYSICS LETTERS
as fuily allowed transitions [21-23 ] _ It is often possible to distinguish these three types of transitions by measuring the dependence on scattering angle (0) and impact energy (Eo) of the inelastic scattering intensity [21-231. In the present experiments on hexatriene, energyloss spectra and relative differential cross sections (DCS) for elastic and inelastic scattering were measured at impact energies (Eo) of 20 eV and 40 eV, and at scattering angles from 0” to 80”. Additional spectra were obtained at impact energies of 50 eV and 75 eV. Most spectra covered the energy-loss range from -0.3 eV to +9.7 eV, but a few spectra extended the observed excitation energy region to 12 eV. The energy resolution of the spectrometer was set typically in the range 0.10-0.15 eV, as determined from the full width at haIf maximum (fwhm) of the peak corresponding to the elastically scattered electrons. However, some of the spectra at low scattering angIes were obtained at a higher resolution, 0.05 eV fwhm. The sample pressure in the collision chamber was 3-4 mtorr, as measured with an uncahbrated Granville-Phillips (SchulzPhelps type) ionization gauge. The hexatriene sample was obtained from the Aldrich Chemical Company, and had a stated purity of 99%. Gavin et al. [5] have reported that the Aldrich sampIes are cis-trans mixtures, containing 60% of the tram isomer. Post et al. [l l] reported that their Aldrich sample was approximately 90% tram hexatriene. Ln the present paper, we will use the state symmetry designations appropriate for the tram isomer, for the sake of convenience_ The samples were vacuum degassed prior to use by means of repeated liquid nitrogen freeze-pump-thaw cycles. The precision of the maximum intensity transition energies which are listed in table 1 and which are discussed below is estimated to be 0.05 eV, for most transitions observed. The Franck-Condon limits given for the two lowest energy transitions have an uncertainty of approximately 0.1 eV. It shodd be noted that the onset of absorption of a band system does not necessadly correspond to the O-O band of the electronic transition. Figs. l-3 display energy loss spectra of hexatriene extending over different parts of the measured energyloss range. The lowest energy transition which we observe has a maximum intensity at 2.61 eV, with a Franck-Condon region that extends from 1.9 eV to 3.5 eV (fig. 1). The relative DCS for the transition
I February 1977
(fig. 4) varies by less than 2 factor of 2 over the scattering angle range loo--80”, at impact energies of both 20 eV and 40 eV. Such behavior is highly characteristic of spin-forbidden transitions [4,2 l-23 J, and confirms the identity of this feature as a singlet + triplet excitation. Semi-empirical calculations [6,24] predict that the lowe$lspin-forbidden transition in tram hexatriene is the X A, + 1 3B, excitation, and that its vertical transition energy is 2.66 eV [24] or 2.40 eV [6], in very good agreement with the present results. This feature has also been observed in the threshold electron-impact spectrum [6], and in optical absorption enhanced by both oxygen [25] and methylene iodide [7] _ The second observed transition in hexatriene (frg. I) peaks at 4.11 eV, and has a Franck-Condon region which extends from 3.6 eV to beyond 4.6 eV. As with the 2.61 eV feature, the relative DCS of the second feature is relatively isotropic in the ang;llar range 20”80” (fig. 4). In addition, the ratio of the intensity of this feature to that of the optically allowed one at 5.13 eV is larger at 20 eV impact energy than at 40 eV, for all scattering angles measured. This increase in relative intensity at lower impact energies is also characteristic of spin-forbidden excitations [4,21-231. These data permit us to assign the 4.11 eV transition as the second singlet + triplet excitation in hexatriene. The calculated values for the z t Ag + 1 3Ag vertical transition energy are 4.13 eV [24], and 3.87 eV [6], in very good agreement with the experimental value. This spin-forbidden excitation correlates well with the feature observed at about 4.2 eV in the threshold electron-impact spectrum [6]. We find no evidence in our spectra for the existence of any transitions between the X ‘As + 1 3A8 and ElA g + 1 i B, excitations (figs. 1 and 2). In particrrlar, we do not observe the very weak feature which Post et al. [ Ii ] reported in the 4.15-4.6 eV region of their E,, = 100 eV electron impact spectra. Et has been suggested [ 11,121 that this weak feature may correspond to the symmetry-forbidden z t Ag + 2 I Ag transition, which is predicted by semi-empiricai caIcuIa-. tions [2,6,8,12,14] to be the lowest energy singfet + singlet excitation. The energy resolution in the present study (0.05-0.15 eV) was considerably better than the 0.29 eV resolution of the experiments of Post et d. fl 11; the incident electron energies employed were lower (20-75 eV versus 100 ev); and the scatter@ angles studied extended to larger values (80” versus 493
Volume 45, number 3 Table 1 Excited electronic
states of 1,3.5-hexatrrene
State symmetry or electronic configuration a)
13B,
CHEMICAL PHYSICS LETTERS
.-~-
Vibrational excited 9)
quanta
--
--
Excitation
W
2.6 b) ~4.2 b)
--.-_-
energy (eV)
other electron impact
1 3Ag b,c)
1 February 1977
photon absorption
h)
2.57 B, 2.58 J)
2.61 (1.9-3.5)
‘)
-
4.11 (3.6-4.6)
n,
4.95 m) 5.13 n)
2 ‘Ag(?) d,e)
-
1 ‘B; b,c,e-g)
0
4.934(4.920)
lv3 Iv3 + Iv2
5.7 b) -
5.136(5.120) 5.288 (5.276) 5.338(5.323) 5.493 (5.478) 5.541(5.524) 5.687 (5.682) 5.737(5.724)
6:2 d) -
6.061(6.073) 6.238(6.220) 6.413 (6.403)
% 2~3 + 1~
3v3 3v3 + Iv2 4v3
0
1%
-
Iv2
lv2 +
Iv5
h
6.9
d)
2v2
2v2 + 1ve Is, + Iv3 lq + Iv2 Iv1 + Iv2 + Iv3
(n, Spa) .@ (n, 6pn) g) (a, 7pn) g) superexcited (SES) SES
8.0 9.2
state (SES)
-
a d)
9.6 d) 10.7 d)
present results k,
6.532(6.555) 6.575(6.583) 6.728 6.771(6.774) 6.902 6.924 6.966(6.964) 7.059(7.055) 7.099 (7.09 1) 7.257(7.253)
5.30 m) 5.5 0) 5.7 o) 6.06 6.25 6.42 6.57 6.75 6.93
7.08 7.25
7.463 7.499
7.48
7.76 7.93 8.03 -
7.77 7.93 8.06 9.1 o) 9.7 0) 10.5 o)
tion of the two triplet states and the 1 rBu state, none of the assignments can be considered certain. d)Ref. [llJ. e, Ref. [lZJ. f) Ref. IS]. Assignments for the 1 r Bu state are taken from ref. [5]. All other assignments are from ref. [ 8 1. Only those peaks are listed which, on the basis of intensity and location, appear to correlate best with the present results. Cis isomer bands are listed in parentheses, and do not necessarily correspond to the assignments given for the fruns isomer (columns 1 and 2). All values are from ref. 181, unless otherwise noted. i) This value was measured from a spectrum published in ref. 171. J) Ref. (251. W The precision of the transition energies is 0.05 eV, unless otherwise noted. Q), The values in parentheses refer.to the estimated Franck-Condon limjts. m)The standard deviation (SD.) of the measured values is 0.03 eV. n, SD. = 0.02 eV. O) SD. = 0.10 eV.
a) b) id h)
With the exce Ref. [61.
494
cP Ref. 1241.
CHEMICAL PHYSICS LETTERS
Volume 45, number 3
e =
8~70”
A
I
5
6
-- 1
0’
4,,
3
2
1977
1,3.5-Heramene E. = 2OeV
I. 3.5- HEXLiTRiENE E,= 40eV
u ::
I February
4 AE CeV)
Fig. 1. Electron energy-Ioss spectrum of 1,3,S-hexatriene at an impact energy (Eo) of 40 eV and a scattering angle (8) of 70”; 1.5-6 eV energy-loss region; 3.8 X 10e3 torr samplepressure reading from an uncahbrated Schulz-Phelps ionization gauge. The small peak which appears at about 3.4 eV energy loss did not appear on any other spectra, and it appears to have been caused by a temporary fluctuation in the noise background.
6
140
’
’ 7
’
’ 8
”
9
”
IO
’
1 II
AE (eVl
Fig. 3. Same as fig. 2 for the following experimental conditions: EO = 20 eV; 8 = 14”; 5.8-I 1 eV ener_@oss region; 4.0 X 10s3 torr sample pressure; 130 meV resolution (fv+hm). 1
.
1
1
a
t
.
I, 3.5 -
i
E.
Herolnene = 20eV
28”). These conditions were selected so as to optimize the probability of detecting forbidden transitions. Furthermore, we obtained spectra in the energy-loss range 3.5 eV to 6 eV involving data accumulation times up to about 30 minutes per 0.1 eV energy-loss range at impact ener@es of 50 eV and 75 eV. This scan time is about one order of magnitude longer than what we normally use. These long observation times were chosen 10000
II
I
1.3.5-Hexafnene E,=
1
40eV
e = 00
,aa IO
AE (eV)
Fig. 2. Same as fig. 1 for the following experimental conditions: Eo = 40 eV; 8 = 0”; 4-9 eV energy-loss region; 3.9 mtorr sample pressure; 0.052 eV resolution (fwhm). The lowest molecular ionization potential (IP) is indicated.
70 9 (D& Fig. 4. Relative differential cross sections (DCS) at an impact enera of 20 eV for elastic scattering (+) and transitions to the 1 3Bu(~), 1 3Ag(o) and 1 r Bu(n) excited states of L,3.5hexatriene. (State designations appropriate to the rmns isomer are used.) The curves are normalized by setting the elastic DCS at 8 = 40” to 1.0. The DCS dues for ebrstic scattering and for the 2 *Ag -, I ‘Bu transition were muitipbed by a factor of 0.1 before plotting. 30
Volume 45, number 3
CHEMICAL
PHYSICSLETTERS
in a special effort to detect the weak feature in the neighborhood cf 4.4 eV energy loss which was reported by Post et al. [ 1 1] . Even under these especially sensitive conditions, we see no evidence of a shoulder or a break in slope in the energy-loss range of 4.2 eV to 4.6 eV. However, we cannot exclude the possibility of an excitation occurring between 4.6 eV and 4.9 eV which could be buried under the tail of the % 1A, + 11 B, band peaking at 4.95 eV. As can be seen in fig. 2, we observe vibronic bands ;n the z 1A, + 1 *B, transition at 4.95,5.13, and 5.30 eV. These values agree well with those measured in ultraviolet absorption [5,26]. The assignments of the vibronic bands (v3), which are primarily due to excitation of the C=C stretching mode [I ,3], are listed in table 1. The DCS of this band system is forward peaked (fig_ 4) in contrast to the DCS curves cf the two singlet + triplet excitations. The intensity of the 5.13 eV band system and its forward-peaked nature are consistent with its assignment as an optically allowed transition. We also find faint breaks in slope at approximately 5.5 eV and 5.7 eV (see fig. 2j. Knoop and Oosterhoff [6] also observed a slope break at 5.7 eV in their threshoicJ spectra, and they tentatively assigned it to the X A, + 2 1B, transition. However, on the basis of the analysis of the optical spectrum [S], we believe that the features at 5.5 eV and 5.7 eV in our spectra are vibronic bands of the % 1A, + 1 * B, transition, and that they are due primarily to excitation of additional quanta of the v3 vibration (see table 1). Between 6.0 eV and 7.4 eV we observe eight features (fig. 2). This is considerably more structure than was observed in either of the other electron-impact experiments [6,11]. However, comparison of our data with W absorption [8] is made difficult because of the fact that the present work was performed on a mixture of tram and cis isomers. Nevertheless, the peak locations of the present results can be roughly correlated with a superposition of the most intense UV peaks of the separate fratzs and cis isomers (table 1). None of the transitions in this region has been given a definitive assignment, alrhcugh semi-empirical calculations [8,12] indicate that excitations to the 3 ‘Ag, 2 1B,, and 3 1Bu stares, as well as u + A* transitions contribute to the observed features. It also seems likely, on the basis of recent spectroscopic work on 1,3butadiene [13,15], that rr - 3p and A + 4p Rydberg 496
1 February 1977
transitions may occur in the 6.0-7.4 eV region of the hexatriene spectrum. Finally, table 1 also contains a list of the transition energies we have measured of features in the 7.4- 11.O eV energy-loss region (fig. 3). Most of these transitions are probably Rydberg in nature. Some are members of series leading to the lowest ionization potential at 8.3 eV [27], while those at higher excitation energy correspond to promotions of electrons from more deeply buried orbitals. In general, our results confirm those of Post et al. [ 111, who first reported transitions to superexcited states in hexatriene. The interpretation of the experimentally observed hexatriene spectra would be considerably facilitated if ab initio calculations of the quality of those performed for 1,3-butadiene [17,18] were available. In particular, it would be of considerable interest to determine whether a description of hexatriene excited states in terms of valence, diffuse, or Rydberg transitions is obtained. Such information would be useful in determining the value of the “molecules-in-molecules” or exciton-coupling description [28] of the valenceexcited states in polyenes. This approach has been shown [18] to be a conceptually simple, but accurate model of these states in 1,3-butadiene, and it seems worthwhile to determine its applicability to the larger polyene, 1,3,5_hexatriene. To summarize the present investigation, we have obtained low energy, variable angle electron-impact energy-loss spectra of a cis- tram mixture of 1,3,5hexatriene. The angular and impact energy dependence of the scattering intensity has confirmed the existence of two singlet + triplet transitions with maximum intensities at 2.61 eV and 4.11 eV, which correspond to the 2 lAg -+ 1 SB, and x” 1A, + 1 3A transitions, respectively. In contrast to a previous ePectron-impact investigation, we do not detect any singlet + singlet transitions in the energy-loss range 4.2 to 4.6 eV, below the optically allowed ?? 1A, + 1 *B, excitation. Many additional peaks in the 6-l 1 eV energy-loss region are observed, and a correlation between these bands and those observed in ultraviolet absorption spectra is made. The peak locations obtained in this study should be useful for comparison with accurate ab initio calculations of the hexatriene electronic spectrum. We are very grateful to Mr. Robert P. Frueholz for
Volume 45, number 3
CHEMICAL PHYSICS LETTERS
his efforts in obtaining the high sensitivity spectra at 50 eV and 75 eV reported in this paper.
References ill B.S. Hudson and B.E. Kohler. Chem. Phys. Letters 14
PI 131 [41
151 t61 [71 (81 191 t101 1111 1121
(1972) 299. K. Schulten and M. Karplus, Chem. Phys. Letters 14 (1972) 305. T.A. Moore and P.-S. Song, Chem. Phys. Letters 19 (1973) 128. O.A. Mosher, W.M. flicker and A. Kuppermann, Chem. Phys. Letters 19 (1973) 332; J. Chem. Phys. 59 (1973) 6502. R.M. Gavin Jr., S. Risemberg and S.A. Rice, J. Chem. Phys. 58 (1973) 3160. F.W.E. Knoop and L.J. Oosterhoff, Chem. Phys. Letters 22 (1973) 247. N.G. Mmnaard and E. Havinga, Rec. Trav. Chim. 92 (1973) 1179. R.M. Gavin Jr. and S.A. Rice, J. Chem. Phys. 60 (1974) 3231. K. Mandal and T.N. Misra, Chem. Phys. Letters 27 (1974) 57. M.B. Robin, Higher excited states of polyatomic molecules, Vol. 2 (Academic Press, New York, 1975). D.E. Post Jr., W.M. Hetherington III and B. Hudson, Chem. Phys. Letters 35 (1975) 259. M. Karpius, R.M. Gavin Jr. and S.A. Rice, J. Chem. Phys. 63 (1975) 5507.
I February
L977
[131 R.M. McDiarmid, J. Chem. Phys. 64 (1976) 514. 1141 K. Schulten, I. Ohmine and XI. Karplus, i. Chem. Phys. 64 (1976) 4422. [15l P.M. Johnson, J. Chem. Phys. 64 (1976) 4638. I161 WM. Flicker, 0-A. Masher and A. Kuppermann, manuscript in preparation. 1171 S. Shih, R.J. Buenker and SD. Peyerimhoff, Chem. Ph;s. Letters 16 (1972) 244. tl81 R.P. Hostcny, T.H. Dunning Jr., R.R. Gdman, A. Pipano and 1. Shavrtt, J. Chem. Phys. 62 (1975) 4764. t191 R.A. Morton and G.A.J. Pitt, Advan. Enzymol. ReIat. Sub& Biochem. 32 (1969) 97. [201 E.W. Abrahamson and SE. Ostroy, Progr. Biophys. MoI. Biol. 17 (1967? 179. [21 I O.A. hfosher, W.M. Flicker and A. Kuppermann, I. Chem. Phys. 62 (1975) 2600. [ 22) 0-A. Mosber, Ph.D. thesis, California Institute of Technology, Pasadena, California (1975); W.hI. Flicker, Ph.D. thesis, Cahfornia Institute of Technology, Pasadena, Cahfornia (1976). [ 23) A. Kuppermann, J.K. Rice and S. Trajmar, I. Phys. Chem. 72 (1968) 3894; S. Trajmar, J.K. Rice and A. Kuppermann, Advan. Chent. Phys. 18 (1970) 15. [24] N.L. Allinger, J.C. Tai and T.W. Stuart, Theoret. Chim. Acta 8 (1967) 101. [25] D.F. Evans, J. Chem. Sot. (1960) 1735. [26] H. Schtiler, E. Lutz and G. Arnold. Spectrochim. AcU 17 (1961) 1043. (271 hf. Beez, G. Bieri, H. Bock and E. Heilbronner, Hehf. Chim. Acta 56 (1973) 1028. [ 281 W-T. Simpson, J. Am. Chem. Sot. 77 (I 955) 6 L64.
497
Volume 45. number 3
1 February
1977
LOW ENERGY, VARIABLE ANGLE ELECTRON-IMPACT EXCITATION OF 1,3,5-HEXATRIENE* Wayne
M.
FLICKER, Oren A. MOSHER$ and Aron KUPPERMANN
Arthur,Amos Noyes Laboratory o/Chemical Pasadena. California 91125. USA Received 3 November
Physics*.
California Institute of Technology,
1976
A mixture of cis and tram 1,3,5-hcxatricne has been studied by electron impact at incident electron energies of 20 cV, 40 eV, 50 eV. and 70 eV, at scattering angles from 0” to 80”, and with effective energy resolutions in the range from 0.05 eV to 0.15 PV. Singlet -L triplet transitions with maximum intensities at 2.61 eV and 4.11 eV are observed. The lowest cncrgy spin-allowed excitation which can be detected is rhc electric dipole-allowed % *A g + 1 ’ Bu transirion (in the notation appropriate for the trans isomer). No evidence has been found for a spin-allowed but symmetry-forbidden z *Ag + 2 *Ag excitation in the vicinity of 4.4 eV transition energy. Many other spin-allowed excitations are observed in the 6- 11 eV energy-loss region, and the correlation between these features and those observed in high resolution ultraviolet absorption spectra and other electron-impact spectra is discussed.
In the past five years, there has been a marked renewal of interest in the electronic spectroscopy of linear polyenes [l-l 81. Of practical concern is the fact that many molecules with important photobiological functions, such as Vitamin A, Vitamin D precursors, and retinal and other carotenes, contain polyene chromophores involving conjugated double bonds. The photophysical pathways by which the absorbed light energy is degraded in these molecules are largely unknown [ 19,20], prim:lrily because until recently there was relatively little information available about the electronic spectroscopy of such molecules. We have previously reported the results of extensive electron-impact investigations of the prototype conjugated polyene, !,3butadiene i4,16], and in this paper, we detail a similar study of 1,3,5hexatriene. Both threshold [6] and high energy [ 1 l] electronimpact investigations of hexatriene have been reported recently, but the limited resolution of the spectra in * Work supported in part by the U.S. Energy Research and Development Administration, Contract No. E(04-3)-767. Report Code: CALT-767P4-149. * Work performed in partial fulftient of the requirements for the Ph.D. degree in Chemistry at the California Institute of Technology. 3 Coniribution No. 5461.
both of these experiments has made it difficult to compare the results quantitatively with high resolution ultraviolet absorption spectra [5,8 1. In the present work, we have obtained variable angle electron-impact spectra at higher resolution than previously reported [6,1 I]. This permits us to clarify the nature of the weak absorptions observed at 4.2 eV [6] and 4.4 eV [ 111. In the conduct of this work, we were most interested in determining whether the lowest energy singlet + singlet transition in hexatriene is electric dipole-allowed, as most experimental data indicate [5-81, or whether it is electric dipole-forbidden, as recent theoretical calculations predict [2,6,8,12,14]. The spectrometer employed in this investigation has been described in detail previously [21,22] _The experiment consists of directing a collimated, energyselected electron beam onto a target gas, contained wzthin a flexible-bellows collision chamber. The kinetic energy distribution of the scattered electrons, at a preselected scattering angle, is then determined by sweeping repeatedly the energy-loss range of interest. The measured electron energy distribution, or energy-loss spectrum, corresponds to a photon absorption spectrum, except that it frequently contains appreciable contributions from both spin-forbidden and spinallowed but electric dipole-forbidden processes, as well
Volume45, number3
CHEMICAL PHYSICS LETTERS
as fuily allowed transitions [21-23 ] _ It is often possible to distinguish these three types of transitions by measuring the dependence on scattering angle (0) and impact energy (Eo) of the inelastic scattering intensity [21-231. In the present experiments on hexatriene, energyloss spectra and relative differential cross sections (DCS) for elastic and inelastic scattering were measured at impact energies (Eo) of 20 eV and 40 eV, and at scattering angles from 0” to 80”. Additional spectra were obtained at impact energies of 50 eV and 75 eV. Most spectra covered the energy-loss range from -0.3 eV to +9.7 eV, but a few spectra extended the observed excitation energy region to 12 eV. The energy resolution of the spectrometer was set typically in the range 0.10-0.15 eV, as determined from the full width at haIf maximum (fwhm) of the peak corresponding to the elastically scattered electrons. However, some of the spectra at low scattering angIes were obtained at a higher resolution, 0.05 eV fwhm. The sample pressure in the collision chamber was 3-4 mtorr, as measured with an uncahbrated Granville-Phillips (SchulzPhelps type) ionization gauge. The hexatriene sample was obtained from the Aldrich Chemical Company, and had a stated purity of 99%. Gavin et al. [5] have reported that the Aldrich sampIes are cis-trans mixtures, containing 60% of the tram isomer. Post et al. [l l] reported that their Aldrich sample was approximately 90% tram hexatriene. Ln the present paper, we will use the state symmetry designations appropriate for the tram isomer, for the sake of convenience_ The samples were vacuum degassed prior to use by means of repeated liquid nitrogen freeze-pump-thaw cycles. The precision of the maximum intensity transition energies which are listed in table 1 and which are discussed below is estimated to be 0.05 eV, for most transitions observed. The Franck-Condon limits given for the two lowest energy transitions have an uncertainty of approximately 0.1 eV. It shodd be noted that the onset of absorption of a band system does not necessadly correspond to the O-O band of the electronic transition. Figs. l-3 display energy loss spectra of hexatriene extending over different parts of the measured energyloss range. The lowest energy transition which we observe has a maximum intensity at 2.61 eV, with a Franck-Condon region that extends from 1.9 eV to 3.5 eV (fig. 1). The relative DCS for the transition
I February 1977
(fig. 4) varies by less than 2 factor of 2 over the scattering angle range loo--80”, at impact energies of both 20 eV and 40 eV. Such behavior is highly characteristic of spin-forbidden transitions [4,2 l-23 J, and confirms the identity of this feature as a singlet + triplet excitation. Semi-empirical calculations [6,24] predict that the lowe$lspin-forbidden transition in tram hexatriene is the X A, + 1 3B, excitation, and that its vertical transition energy is 2.66 eV [24] or 2.40 eV [6], in very good agreement with the present results. This feature has also been observed in the threshold electron-impact spectrum [6], and in optical absorption enhanced by both oxygen [25] and methylene iodide [7] _ The second observed transition in hexatriene (frg. I) peaks at 4.11 eV, and has a Franck-Condon region which extends from 3.6 eV to beyond 4.6 eV. As with the 2.61 eV feature, the relative DCS of the second feature is relatively isotropic in the ang;llar range 20”80” (fig. 4). In addition, the ratio of the intensity of this feature to that of the optically allowed one at 5.13 eV is larger at 20 eV impact energy than at 40 eV, for all scattering angles measured. This increase in relative intensity at lower impact energies is also characteristic of spin-forbidden excitations [4,21-231. These data permit us to assign the 4.11 eV transition as the second singlet + triplet excitation in hexatriene. The calculated values for the z t Ag + 1 3Ag vertical transition energy are 4.13 eV [24], and 3.87 eV [6], in very good agreement with the experimental value. This spin-forbidden excitation correlates well with the feature observed at about 4.2 eV in the threshold electron-impact spectrum [6]. We find no evidence in our spectra for the existence of any transitions between the X ‘As + 1 3A8 and ElA g + 1 i B, excitations (figs. 1 and 2). In particrrlar, we do not observe the very weak feature which Post et al. [ Ii ] reported in the 4.15-4.6 eV region of their E,, = 100 eV electron impact spectra. Et has been suggested [ 11,121 that this weak feature may correspond to the symmetry-forbidden z t Ag + 2 I Ag transition, which is predicted by semi-empiricai caIcuIa-. tions [2,6,8,12,14] to be the lowest energy singfet + singlet excitation. The energy resolution in the present study (0.05-0.15 eV) was considerably better than the 0.29 eV resolution of the experiments of Post et d. fl 11; the incident electron energies employed were lower (20-75 eV versus 100 ev); and the scatter@ angles studied extended to larger values (80” versus 493
Volume 45, number 3 Table 1 Excited electronic
states of 1,3.5-hexatrrene
State symmetry or electronic configuration a)
13B,
CHEMICAL PHYSICS LETTERS
.-~-
Vibrational excited 9)
quanta
--
--
Excitation
W
2.6 b) ~4.2 b)
--.-_-
energy (eV)
other electron impact
1 3Ag b,c)
1 February 1977
photon absorption
h)
2.57 B, 2.58 J)
2.61 (1.9-3.5)
‘)
-
4.11 (3.6-4.6)
n,
4.95 m) 5.13 n)
2 ‘Ag(?) d,e)
-
1 ‘B; b,c,e-g)
0
4.934(4.920)
lv3 Iv3 + Iv2
5.7 b) -
5.136(5.120) 5.288 (5.276) 5.338(5.323) 5.493 (5.478) 5.541(5.524) 5.687 (5.682) 5.737(5.724)
6:2 d) -
6.061(6.073) 6.238(6.220) 6.413 (6.403)
% 2~3 + 1~
3v3 3v3 + Iv2 4v3
0
1%
-
Iv2
lv2 +
Iv5
h
6.9
d)
2v2
2v2 + 1ve Is, + Iv3 lq + Iv2 Iv1 + Iv2 + Iv3
(n, Spa) .@ (n, 6pn) g) (a, 7pn) g) superexcited (SES) SES
8.0 9.2
state (SES)
-
a d)
9.6 d) 10.7 d)
present results k,
6.532(6.555) 6.575(6.583) 6.728 6.771(6.774) 6.902 6.924 6.966(6.964) 7.059(7.055) 7.099 (7.09 1) 7.257(7.253)
5.30 m) 5.5 0) 5.7 o) 6.06 6.25 6.42 6.57 6.75 6.93
7.08 7.25
7.463 7.499
7.48
7.76 7.93 8.03 -
7.77 7.93 8.06 9.1 o) 9.7 0) 10.5 o)
tion of the two triplet states and the 1 rBu state, none of the assignments can be considered certain. d)Ref. [llJ. e, Ref. [lZJ. f) Ref. IS]. Assignments for the 1 r Bu state are taken from ref. [5]. All other assignments are from ref. [ 8 1. Only those peaks are listed which, on the basis of intensity and location, appear to correlate best with the present results. Cis isomer bands are listed in parentheses, and do not necessarily correspond to the assignments given for the fruns isomer (columns 1 and 2). All values are from ref. 181, unless otherwise noted. i) This value was measured from a spectrum published in ref. 171. J) Ref. (251. W The precision of the transition energies is 0.05 eV, unless otherwise noted. Q), The values in parentheses refer.to the estimated Franck-Condon limjts. m)The standard deviation (SD.) of the measured values is 0.03 eV. n, SD. = 0.02 eV. O) SD. = 0.10 eV.
a) b) id h)
With the exce Ref. [61.
494
cP Ref. 1241.
CHEMICAL PHYSICS LETTERS
Volume 45, number 3
e =
8~70”
A
I
5
6
-- 1
0’
4,,
3
2
1977
1,3.5-Heramene E. = 2OeV
I. 3.5- HEXLiTRiENE E,= 40eV
u ::
I February
4 AE CeV)
Fig. 1. Electron energy-Ioss spectrum of 1,3,S-hexatriene at an impact energy (Eo) of 40 eV and a scattering angle (8) of 70”; 1.5-6 eV energy-loss region; 3.8 X 10e3 torr samplepressure reading from an uncahbrated Schulz-Phelps ionization gauge. The small peak which appears at about 3.4 eV energy loss did not appear on any other spectra, and it appears to have been caused by a temporary fluctuation in the noise background.
6
140
’
’ 7
’
’ 8
”
9
”
IO
’
1 II
AE (eVl
Fig. 3. Same as fig. 2 for the following experimental conditions: EO = 20 eV; 8 = 14”; 5.8-I 1 eV ener_@oss region; 4.0 X 10s3 torr sample pressure; 130 meV resolution (fv+hm). 1
.
1
1
a
t
.
I, 3.5 -
i
E.
Herolnene = 20eV
28”). These conditions were selected so as to optimize the probability of detecting forbidden transitions. Furthermore, we obtained spectra in the energy-loss range 3.5 eV to 6 eV involving data accumulation times up to about 30 minutes per 0.1 eV energy-loss range at impact ener@es of 50 eV and 75 eV. This scan time is about one order of magnitude longer than what we normally use. These long observation times were chosen 10000
II
I
1.3.5-Hexafnene E,=
1
40eV
e = 00
,aa IO
AE (eV)
Fig. 2. Same as fig. 1 for the following experimental conditions: Eo = 40 eV; 8 = 0”; 4-9 eV energy-loss region; 3.9 mtorr sample pressure; 0.052 eV resolution (fwhm). The lowest molecular ionization potential (IP) is indicated.
70 9 (D& Fig. 4. Relative differential cross sections (DCS) at an impact enera of 20 eV for elastic scattering (+) and transitions to the 1 3Bu(~), 1 3Ag(o) and 1 r Bu(n) excited states of L,3.5hexatriene. (State designations appropriate to the rmns isomer are used.) The curves are normalized by setting the elastic DCS at 8 = 40” to 1.0. The DCS dues for ebrstic scattering and for the 2 *Ag -, I ‘Bu transition were muitipbed by a factor of 0.1 before plotting. 30
Volume 45, number 3
CHEMICAL
PHYSICSLETTERS
in a special effort to detect the weak feature in the neighborhood cf 4.4 eV energy loss which was reported by Post et al. [ 1 1] . Even under these especially sensitive conditions, we see no evidence of a shoulder or a break in slope in the energy-loss range of 4.2 eV to 4.6 eV. However, we cannot exclude the possibility of an excitation occurring between 4.6 eV and 4.9 eV which could be buried under the tail of the % 1A, + 11 B, band peaking at 4.95 eV. As can be seen in fig. 2, we observe vibronic bands ;n the z 1A, + 1 *B, transition at 4.95,5.13, and 5.30 eV. These values agree well with those measured in ultraviolet absorption [5,26]. The assignments of the vibronic bands (v3), which are primarily due to excitation of the C=C stretching mode [I ,3], are listed in table 1. The DCS of this band system is forward peaked (fig_ 4) in contrast to the DCS curves cf the two singlet + triplet excitations. The intensity of the 5.13 eV band system and its forward-peaked nature are consistent with its assignment as an optically allowed transition. We also find faint breaks in slope at approximately 5.5 eV and 5.7 eV (see fig. 2j. Knoop and Oosterhoff [6] also observed a slope break at 5.7 eV in their threshoicJ spectra, and they tentatively assigned it to the X A, + 2 1B, transition. However, on the basis of the analysis of the optical spectrum [S], we believe that the features at 5.5 eV and 5.7 eV in our spectra are vibronic bands of the % 1A, + 1 * B, transition, and that they are due primarily to excitation of additional quanta of the v3 vibration (see table 1). Between 6.0 eV and 7.4 eV we observe eight features (fig. 2). This is considerably more structure than was observed in either of the other electron-impact experiments [6,11]. However, comparison of our data with W absorption [8] is made difficult because of the fact that the present work was performed on a mixture of tram and cis isomers. Nevertheless, the peak locations of the present results can be roughly correlated with a superposition of the most intense UV peaks of the separate fratzs and cis isomers (table 1). None of the transitions in this region has been given a definitive assignment, alrhcugh semi-empirical calculations [8,12] indicate that excitations to the 3 ‘Ag, 2 1B,, and 3 1Bu stares, as well as u + A* transitions contribute to the observed features. It also seems likely, on the basis of recent spectroscopic work on 1,3butadiene [13,15], that rr - 3p and A + 4p Rydberg 496
1 February 1977
transitions may occur in the 6.0-7.4 eV region of the hexatriene spectrum. Finally, table 1 also contains a list of the transition energies we have measured of features in the 7.4- 11.O eV energy-loss region (fig. 3). Most of these transitions are probably Rydberg in nature. Some are members of series leading to the lowest ionization potential at 8.3 eV [27], while those at higher excitation energy correspond to promotions of electrons from more deeply buried orbitals. In general, our results confirm those of Post et al. [ 111, who first reported transitions to superexcited states in hexatriene. The interpretation of the experimentally observed hexatriene spectra would be considerably facilitated if ab initio calculations of the quality of those performed for 1,3-butadiene [17,18] were available. In particular, it would be of considerable interest to determine whether a description of hexatriene excited states in terms of valence, diffuse, or Rydberg transitions is obtained. Such information would be useful in determining the value of the “molecules-in-molecules” or exciton-coupling description [28] of the valenceexcited states in polyenes. This approach has been shown [18] to be a conceptually simple, but accurate model of these states in 1,3-butadiene, and it seems worthwhile to determine its applicability to the larger polyene, 1,3,5_hexatriene. To summarize the present investigation, we have obtained low energy, variable angle electron-impact energy-loss spectra of a cis- tram mixture of 1,3,5hexatriene. The angular and impact energy dependence of the scattering intensity has confirmed the existence of two singlet + triplet transitions with maximum intensities at 2.61 eV and 4.11 eV, which correspond to the 2 lAg -+ 1 SB, and x” 1A, + 1 3A transitions, respectively. In contrast to a previous ePectron-impact investigation, we do not detect any singlet + singlet transitions in the energy-loss range 4.2 to 4.6 eV, below the optically allowed ?? 1A, + 1 *B, excitation. Many additional peaks in the 6-l 1 eV energy-loss region are observed, and a correlation between these bands and those observed in ultraviolet absorption spectra is made. The peak locations obtained in this study should be useful for comparison with accurate ab initio calculations of the hexatriene electronic spectrum. We are very grateful to Mr. Robert P. Frueholz for
Volume 45, number 3
CHEMICAL PHYSICS LETTERS
his efforts in obtaining the high sensitivity spectra at 50 eV and 75 eV reported in this paper.
References ill B.S. Hudson and B.E. Kohler. Chem. Phys. Letters 14
PI 131 [41
151 t61 [71 (81 191 t101 1111 1121
(1972) 299. K. Schulten and M. Karplus, Chem. Phys. Letters 14 (1972) 305. T.A. Moore and P.-S. Song, Chem. Phys. Letters 19 (1973) 128. O.A. Mosher, W.M. flicker and A. Kuppermann, Chem. Phys. Letters 19 (1973) 332; J. Chem. Phys. 59 (1973) 6502. R.M. Gavin Jr., S. Risemberg and S.A. Rice, J. Chem. Phys. 58 (1973) 3160. F.W.E. Knoop and L.J. Oosterhoff, Chem. Phys. Letters 22 (1973) 247. N.G. Mmnaard and E. Havinga, Rec. Trav. Chim. 92 (1973) 1179. R.M. Gavin Jr. and S.A. Rice, J. Chem. Phys. 60 (1974) 3231. K. Mandal and T.N. Misra, Chem. Phys. Letters 27 (1974) 57. M.B. Robin, Higher excited states of polyatomic molecules, Vol. 2 (Academic Press, New York, 1975). D.E. Post Jr., W.M. Hetherington III and B. Hudson, Chem. Phys. Letters 35 (1975) 259. M. Karpius, R.M. Gavin Jr. and S.A. Rice, J. Chem. Phys. 63 (1975) 5507.
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L977
[131 R.M. McDiarmid, J. Chem. Phys. 64 (1976) 514. 1141 K. Schulten, I. Ohmine and XI. Karplus, i. Chem. Phys. 64 (1976) 4422. [15l P.M. Johnson, J. Chem. Phys. 64 (1976) 4638. I161 WM. Flicker, 0-A. Masher and A. Kuppermann, manuscript in preparation. 1171 S. Shih, R.J. Buenker and SD. Peyerimhoff, Chem. Ph;s. Letters 16 (1972) 244. tl81 R.P. Hostcny, T.H. Dunning Jr., R.R. Gdman, A. Pipano and 1. Shavrtt, J. Chem. Phys. 62 (1975) 4764. t191 R.A. Morton and G.A.J. Pitt, Advan. Enzymol. ReIat. Sub& Biochem. 32 (1969) 97. [201 E.W. Abrahamson and SE. Ostroy, Progr. Biophys. MoI. Biol. 17 (1967? 179. [21 I O.A. hfosher, W.M. Flicker and A. Kuppermann, I. Chem. Phys. 62 (1975) 2600. [ 22) 0-A. Mosber, Ph.D. thesis, California Institute of Technology, Pasadena, California (1975); W.hI. Flicker, Ph.D. thesis, Cahfornia Institute of Technology, Pasadena, Cahfornia (1976). [ 23) A. Kuppermann, J.K. Rice and S. Trajmar, I. Phys. Chem. 72 (1968) 3894; S. Trajmar, J.K. Rice and A. Kuppermann, Advan. Chent. Phys. 18 (1970) 15. [24] N.L. Allinger, J.C. Tai and T.W. Stuart, Theoret. Chim. Acta 8 (1967) 101. [25] D.F. Evans, J. Chem. Sot. (1960) 1735. [26] H. Schtiler, E. Lutz and G. Arnold. Spectrochim. AcU 17 (1961) 1043. (271 hf. Beez, G. Bieri, H. Bock and E. Heilbronner, Hehf. Chim. Acta 56 (1973) 1028. [ 281 W-T. Simpson, J. Am. Chem. Sot. 77 (I 955) 6 L64.
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