Excitation Spectra of 9,10-DCA

The excitation spectrum of 9,10-DCA in helium droplets is presented in fig. 5.11 with ν0 = 25889 cm⁻¹ . The most intense transition exhibits the lowest transition energy and is thus assigned to the electronic origin. The solvent shift of about 60 cm⁻¹ to the red compared to the gas phase [AHJ88] is in the typical range observed for rigid molecules doped into helium droplets. The variations of vibrational frequencies compared to the gas phase are also in the typical range of less than 4 %. (table 5.3) As discussed in chapter 4.2 the intensity pattern in the excitation spectrum of 9,10-DCA in helium droplets is practically identical to the pattern in the absorption spectrum, but differs significantly from the pattern in the fluorescence excitation spectrum in the gas phase.

Fig. 5.11: Excitation spectrum of 9,10-DCA in helium droplets with ν₀ = 25889 cm⁻¹ nor-malized to laser intensity. The spectrum in the inset is recorded with higher laser intensity. In both spectra the laser intensity was low enough to avoid saturation effects.

All electronic transitions in the droplet spectrum reveal the same fine structure as can be seen in fig.s 5.12, 5.13 and 5.14 and which is entirely absent in the gas phase. Fig. 5.12 shows the electronic origin recorded with different laser intensities. At laser intensities lower than used for the solid line spectrum the pattern does not change whereas at higher laser intensities (dotted line) the relative intensity of the peak lowest in energy (at ν₀) decreases. As discussed in chapter 4 this indicates that only the peak at ν₀ = 25889 cm⁻¹ can be assigned to a ZPL. All other transitions to its blue side belong to the accompanying PW, in particular the sharp features corresponding to phonon energies of 0.8, 2.6, and 4.5 cm⁻¹ .

A fine structure could also be due to the abundance of different isotopomers of 9,10-DCA in the sample. The natural abundance ratio of the isotopes³⁵Cl and³⁷Cl amounts to 3:1 and thus the isotopomers (³⁵Cl,³⁵Cl)-AN, (³⁵Cl,³⁷Cl)-AN, (³⁷Cl,³⁷Cl)-AN are expected in an abundance ratio of 9:6:1. [AEJ82] The different saturation behavior and the intensity ratio of the sharp transitions indicate that the fine structure observed at the electronic origin is not due to different isotopomers of 9,10-DCA. Isotopic spectral shifts are obser-ved for vibronic transitions inducing vibrational motion of the Cl-atoms. In particular

Fig. 5.12: Excitation spectrum of 9,10-DCA in helium droplets at the electronic origin recor-ded with different laser intensities.(see text) ν₀= 25889 cm⁻¹ .

the 305 cm⁻¹ vibration shown in fig. 5.13 exhibits a marked Cl isotope effect. [AEJ82]

Three distinct transitions with an intensity ratio matching with the abundance ratio of the three isotopomers appear. The vibration is assigned to the symmetric C-Cl stretching vibration involving motion of both Cl-atoms. [CG82, AEJ82] The equidistant spacings of these vibronic transitions of the three different isotopomers amounts to 3.2 cm⁻¹ as found in the gas phase. [AEJ82] The C-Cl stretching mode can also be found as overto-ne (610 cm⁻¹ ) and in various combinations e.g. with 403, 642, 805 (2×403), 1168 and 1390 cm⁻¹ vibrations as shown in fig.s 5.13 and 5.14. Most important, the transitions of each isotopomer spectrally separated at the vibronic transitions involving the C-Cl stretching mode exhibit the same fine structure as the electronic origin. (cf. fig.s 5.12, 5.13, 5.14) The reappearance of this fine structure at the isotopically split vibronic bands provides further evidence that it is not due to different isotopomers.

The isotopic spectral shift at the electronic origin is due to the differences in the zero-point energy between the electronic ground and excited state for the different isotopo-mers. However, the vibrational frequencies in both electronic states hardly differ and exchange of the Cl-isotopes has only a minor effect on the zero-point energies due to the mass ratio of the different isotopes close to one. Thus, the isotopic spectral shift at the electronic origin, and many other vibronic transitions not involving excitation of significant motion of Cl-atoms, remains hidden below the corresponding line shape. (cf.

fig.s 5.12, 5.13 and 5.14)

Fig. 5.13: Excitation spectra of selected vibronic bands of 9,10-DCA in helium droplets on an expanded scale.ν0 = 25889 cm⁻¹ . Transitions on the right side are assigned to combinations of the transitions on the left side with the C-Cl stretching mode.

Fig. 5.14: Excitation spectra of selected vibronic bands of 9,10-DCA in helium droplets on an expanded scale. ν0 = 25889 cm⁻¹ . Note the different wavenumber scaling for the lower sections. The line shape of the vibronic transition atν−ν0 = 206 cm⁻¹ was fitted with a Lorentzian with a full width of 8 cm⁻¹ . (see text)

Interestingly, the vibronic transition shown fig. 5.14 with a vibrational energy of 206 cm⁻¹ in S1 has a much larger line width of almost 10 cm⁻¹ than all other transitions with FWHMs of typically < 1 cm⁻¹ . In the gas phase spectrum the line width of the cor-responding transition is similar to that of the other transitions and the fluorescence quantum yield of the excited state is close to one. [AHJ88] Thus, the line broadening is due to the helium environment. The line shape can be fitted with a Lorentzian function with a line width of 8 cm⁻¹ indicating a decay time of 0.7 ps due to damping of the vibronic state. For AN a weak low energy mode with 209 cm⁻¹ in the gas phase spectrum (cf. chapter 5.1) was absent in the corresponding droplet spectrum. (cf. chapter 5.1) In contrast to AN (cf. chapter 5.1), 9-MA (cf. chapter 5.6), and 1-MA (cf. chapter 5.7) no signal red shifted to the electronic origin due to complexation with an impurity could be found even with high laser intensities.

Tab. 5.3: Transition wavenumbersν−ν₀ (cm⁻¹ ) in the excitation spectrum of 9,10-DCA in helium droplets and relative peak intensities I/I₀. All numbers are referred to the ZPL at the electronic origin (ν0 = 25889 cm⁻¹ , I0 = 1) and correspond to vibra-tional frequencies in the electronically excited state. The sharp features of the fine structure of each vibronic transitions are not listed. For comparison gas phase data from the literature [AHJ88], ν_vib(jet), are listed. However, only frequencies of the most intense transitions of 9,10-DCA are given in the literature and the splitting due to various isotopomers is not listed.

ν−ν₀ / cm⁻¹ I/I₀ ν_vib / cm⁻¹ shift / cm⁻¹

ν−ν₀ / cm⁻¹ I/I₀ ν_vib / cm⁻¹ shift / cm⁻¹

The dispersed emission spectrum of 9,10-DCA recorded upon excitation at the electronic origin is shown in fig. 5.15. The intensity pattern and frequency positions of the transiti-ons are independent of the excitation frequency. Except for a solvent shift the spectrum matches with the emission spectrum recorded in the gas phase upon excitation at the electronic origin. [TYH⁺89]

The differences in the vibrational frequencies in helium droplets and in the gas phase (table 5.6) are to a large extent due to the low spectral resolution and consequently large experimental uncertainties in the vibrational frequencies in both experiments. Note that the intense 1263 cm⁻¹ vibration in the droplet spectrum is not listed in ref. [TYH⁺89]

but is clearly visible in their spectrum.

The electronic origin in the emission spectrum recorded upon excitation at the electro-nic origin was identified at 25893 cm⁻¹ which is in agreement with the frequency of the electronic origin in the excitation spectrum at 25889 cm⁻¹ within experimental accuracy.

However, the emission spectrum exhibits a gradual shift to the red increasing with excess excitation energy. Fig. 5.16 shows parts of the emission spectrum on an expanded scale recorded upon excitation with vibrational excess energies as indicated. Fig. 5.16(a) and (b) demonstrate the gradual shift at the electronic origin while fig. 5.16(c) and (d) reveal the influence on some of the prominent modes. The emission spectrum recorded upon excitation with 1168 cm⁻¹ vibrational excess energy is shifted by about 10 ± 3 cm⁻¹