Excitation Spectra of 8-PhPM

The excitation spectrum of 8-PhPM recorded in helium droplets is shown in fig. 7.9 (a) in comparison to the excitation spectrum of the free molecule in the gas phase in fig. 7.9 (b). The droplet spectrum appears broader than the gas phase spectrum though roughly describes its envelope. Thus, together with the fact that no signal is observed further to the red, the transition at 20390 cm⁻¹ is assigned to the electronic origin, corresponding to a solvent shift of 28 cm⁻¹ to the red compared to the gas phase.

Fig. 7.9: Excitation spectra of 8-PhPM in helium droplets (a) and in the supersonic jet (b) withν0 as indicated. The laser intensity was low enough to avoid saturation effects.

Both spectra were normalized to the laser intensity.

The excitation spectrum in fig. 7.9 (a) shows only one sharp transition with a line width of about 0.4 cm⁻¹ at the electronic origin. Decreasing the oven temperature does not affect the shape of the spectrum.

An enlargement of the excitation spectrum at the electronic origin recorded with dif-ferent laser intensities is presented in fig. 7.10. Lowering the laser intensity does not

Fig. 7.10: Excitation spectrum of 8-PhPM in helium droplets recorded with different laser intensities.(see text) ν₀= 20390 cm⁻¹ .

alter its shape compared to the solid line spectrum but only decreases its intensity.

Thus, saturation effects can be excluded to cause the broadening. Increasing the laser intensity by about an order of magnitude slightly decreases the relative intensity of the sharp transition. This reflects a different saturation behavior of the leading peak and the broad feature to its blue side. Together with the fact that the broad contribution has no counterpart in the gas phase spectrum the saturation behavior is indicative for a sharp ZPL accompanied by an intense PW. The PW appears as a broad asymmetric feature similar as observed for fluorazene (FPP) in helium droplets discussed in chapter 6. However, in contrast to the spectrum of FPP a sharp ZPL is observed only at the electronic origin. Further, not all of the broad bands in the spectrum shown in fig. 7.9 (a) resemble the same asymmetric broad line shape.

In particular, the steep rise and asymmetric tail observed at the electronic origin reap-pears only at the fundamental and first overtone of a 217 cm⁻¹ mode, which is also observed in the supersonic jet with a vibrational frequency of 222 cm⁻¹ . In contrast, the other bands reveal a rather symmetric spectral shape. The peak intensities of the first three bands following the electronic origin appear shifted by about 80 cm⁻¹ , 140 cm⁻¹ and 195 cm⁻¹ from the electronic origin. These bands are assigned to the members of the progression of the torsional mode. The intensity pattern of the progression indicates different equilibrium positions in the torsional potential in S₀ and S₁.

The S/N ratio and the drastic line broadening do not allow to resolve any fine structure of the vibrational satellites in the excitation spectrum. Due to the low signal level recording dispersed emission spectra was also not possible .

7.2.2 Discussion

The excitation spectrum of the phenylated PM dye 8-PhPM appears broadened com-pared to its spectrum in the gas phase. A similar broadening was observed for meso-phenylated anthracene (9-PA). (cf. chapter 5.5) In case of 9-PA the broadening could be attributed to a damping of the nuclear rearrangement induced upon excitation. This lead to a homogeneous broadening of all transitions in the excitation spectrum as dis-cussed in chapter 5.9.1. However, in the spectrum of 8-PhPM different spectral shapes are observed. The electronic origin as well as the fundamental and the first overtone of a 217 cm⁻¹ mode exhibit an intense and broad asymmetric band typical for a PW.

These bands exhibit a steep rise on the red side and a more moderate decline on the blue side. They were tentatively fitted with the phenomenological function

y=







0, forν < νc

B(ν−ν_c)exp(−

√ν−ν_c

C ), forν ≥ν_c

(7.1)

with ν_c the frequency of the onset of the broad bands as shown in fig. 7.11 for the electronic origin. Therein, a Lorentzian function was added to describe the ZPL. The parameters B and C in fig. 7.11 were 14.5 cm and 1.35 cm⁻¹ , respectively.

Similarly, the PW of pyrene observed in the excitation spectrum in helium droplets was simulated with an exponential decay as reported in ref. [RKH⁺04]. Therein, the steep rise was assumed to be vertical and the PW to be separated from the ZPL by 5 cm⁻¹ .

Fig. 7.11: Fit of the excitation spectrum of 8-PhPM in helium droplets at the electronic origin. (see text)ν0 = 20390 cm⁻¹ .

The excitation spectrum of 8-PhPM in the gas phase for excess energies ν −ν₀ be-low 200 cm⁻¹ mainly consists of the harmonic progression of the phenyl torsion. Each transition of this progression is additionally coupled to the anharmonic low-frequency progression with transitions at 15.5 cm⁻¹ , 26.9 cm⁻¹, 43.0 cm⁻¹, and 63.7 cm⁻¹. [SF09]

This anharmonic low-frequency progression was also present in the gas phase spectrum of the parent compound BDP though was completely missing in the droplet spectrum (cf. chapter 7.1). Assuming this low-frequency mode to be absent also in the droplet spectrum of 8-PhPM, it consists mainly of the progression of the phenyl torsion built on the electronic origin and the 217 cm⁻¹ mode. Fig. 7.12 shows a simulation (blue) of the progression of the phenyl torsion built on the electronic origin. This pattern repeats coupled to the fundamental and the first overtone of the 217 cm⁻¹ mode. The simula-tion used is a linear combinasimula-tion of equ. 7.1 to describe the first broad band and four Lorentzians. One of them describes the sharp ZPL at the electronic origin. The other Lorentzians have center frequencies of 79.5 cm⁻¹ , 140 cm⁻¹ , and 195 cm⁻¹ with line widths (FWHM) of 36 cm⁻¹ and relative intensities as indicated in fig. 7.12.

The simulation in fig. 7.12 (blue) demonstrates that transitions involving excitations of the torsional mode can roughly be described by Lorentzians. In contrast, the first band is ill-described by a Lorentzian (as well as any other symmetric function such as a Gaussian) function. The line widths of the torsional modes would correspond to a decay time of 0.15 ps similar as determined for 9-PA. (cf. chapter 5.9.1)

Fig. 7.12: Simulation (blue) of the excitation spectrum of 8-PhPM in helium droplets by a linear combination of an exponential and four Lorentzian functions. (see text) The center frequencies and relative intensities of the vibrational satellites are indicated by vertical lines (green). The experimental droplet spectrum (black) is shown for comparison.

To account for other vibronic transitions observed in the supersonic jet the spectrum in helium droplets was further simulated by a linear combination of the function presented in equ. 7.1 to describe the electronic origin and the fundamental and the overtone of the 217 cm⁻¹ and Lorentzians for all others. Fig. 7.13 shows the simulated spectrum (blue) compared to the experimental droplet spectrum (black). The red stick spectrum illustrates the relative peak intensities and frequencies of all transitions observed in the supersonic jet omitting the unidentified low-frequency mode which was absent in helium droplets in the case of BDP.

Fig. 7.13: Simulation (blue) of the excitation spectra of 8-PhPM in helium droplets by a linear combination of an exponential and Lorentzian functions. (see text) The stick spectrum for the simulation (green) and of parts of the experimental gas phase spectrum (red) are also shown. Corresponding transitions of the red and green spectra are labeled with the same number though with an additional prime for the transitions in the green spectrum. The experimental droplet spectrum (black) is shown for comparison.

For the simulation the parameters B and C in equ. 7.1 were fixed to 14.5 cm and 1.25 cm⁻¹ , respectively, for the asymmetric bands with ν −ν₀ = 0 cm⁻¹ , 217 cm⁻¹ and 434 cm⁻¹ . All Lorentzians have the same line width of 32 cm⁻¹ corresponding to a decay time of about 0.2 ps. The center frequencies of the vibrational satellites were altered to obtain better agreement. Corresponding transitions in the gas phase and in

helium droplets have the same number with a prime added to label the droplet transiti-ons. In particular the center frequencies of the torsional mode were altered from 60 cm⁻¹ (1), 117 cm⁻¹ (2), and 174 cm⁻¹ (4) as observed in the gas phase (red) to 78 cm⁻¹ (1’), 142 cm⁻¹ (2’), and 195 cm⁻¹ (4’) (green), respectively, for the fundamental and the two overtones. A similar scaling was used for these modes coupled to the 217 cm⁻¹ mode.

The relative intensities were altered as indicated by the green stick spectrum in fig. 7.13.

The relative intensities used for the PWs at 0 cm⁻¹ , 217 cm⁻¹ and 434 cm⁻¹ cannot be compared with the relative intensities for the Lorentzians.

The members of the progression of the phenyl torsion 1’, 2’ and 4’ in helium droplets are shifted compared to the gas phase frequencies (ν−ν₀) of 1, 2, and 4, respectively, by about 20 cm⁻¹ to higher energies. Thus, the origin of the progression would be expected at ν −ν0 ≈ 20 cm⁻¹ which is in contrast to the observation of a sharp transition at ν−ν₀ = 0 cm⁻¹ readily assigned to the electronic origin. This may indicate that the low frequency anharmonic progression observed in the gas phase spectrum is not absent in the droplet spectrum of 8-PhPM. In this case the peak positions of the broad bands in the droplet spectrum correspond to the center of the anharmonic progression coupled to the members of the torsional progression. This position is separated by about 20 cm⁻¹ from the first transition of the anharmonic progression. (cf. fig. 7.9(b)) In contrast, the electronic origin in the droplet spectrum may correspond to the transition lowest in energy of the anharmonic progression built on the electronic origin. The anharmonic progression may cause the deviations between the simulated (blue) and experimental (black) droplet spectrum in fig. 7.13.

To conclude, the electronic origin and the vibrational satellites of the 217 cm⁻¹ mode not coupled to the phenyl torsion reveal a rather asymmetric broadening typical for the dominance of a PW. This is in agreement with the observed saturation behavior and indicates a strong electron-phonon coupling. All transitions coupled to the phenyl torsion appear broadened in helium droplets. However, it is not clear if the anharmonic low-frequency progression observed in the gas phase is completely absent, as observed for BDP, or if it contributes to the observed band widths. The latter is indicated by the center frequency of the broad bands of the phenyl torsion. Therefore, only a lower limit for the decay time can be estimated to 0.2 ps. Since the broadening does not occur in the gas phase or at the electronic origin it can presumably be attributed to a fast dissipation of vibrational excess energy following the arguments in chapters 7.4 and 7.5.

Regardless of the presence or absence of the anharmonic low-frequency mode in the dro-plet spectrum the electronic origin is the only sharp ZPL that could be resolved. Sharp ZPLs might also be found in the steep rise of the transitions at 217 cm⁻¹ and 434 cm⁻¹ though could not unequivocally be assigned. In view of the S/N-ratio and the relative

peak intensity of the sharp transition at the electronic origin compared to the broad signal in fig. 7.10 a sharp signal with half of the intensity of the sharp origin, as to be expected at 217 cm⁻¹ , would be hidden below the noise.

Interestingly, both broadening mechanisms, namely the dominance of PWs and the dam-ping of excited states, seem to appear in the spectrum of 8-PhPM. The latter dominates for vibronic transitions involving excitation of the phenyl torsion. However, at this stage it is not clear why it dominates over the electron-phonon coupling. If both processes would be affective for the transitions involving the phenyl torsion the line shape could not be described by a symmetric function with a line width similar as observed for the asymmetric PW at the electronic origin.

The driving force for both, electron-phonon coupling based on a variation of the confi-guration of the non-superfluid solvation layer and the damping based on a variation of an intramolecular coordinate (such as a torsional angle), is the change of the electron density induced upon electronic excitation. One might speculate that only upon excita-tion of the torsional mode the damping is faster than the rearrangement of the solvaexcita-tion layer thus dominating the line shape of transitions involving excitation of the torsional mode.