Excitation Spectra of BDP

Fig. 7.2 (a) shows the fluorescence excitation spectrum of BDP in helium droplets with ν₀ = 20504 cm⁻¹ . The most intense transition is assigned to the electronic origin due to the fact that no substantial signal can be observed with a lower transition energy. The weak signal marked with an asterisk is assigned to a complex of BDP with an impurity from the sample since it decreases in intensity with time. Thus, the impurity probably stems from the sample. Signals of the complex probably appear also to the red of the vibronic transitions, though, too weak to be distinguished from the background noise.

The spectrum is dominated by the electronic origin and exhibits few vibronic transitions as is expected for a rigid molecule. The vibrational frequencies are in agreement with those observed in the gas phase. [SF09] However, in the gas phase all of the

respecti-Fig. 7.2: Excitation spectra of BDP in helium droplets (a) and in the supersonic jet (b) withν0 as indicated. The laser intensity was low enough to avoid saturation effects.

Laser intensity was kept constant. Signals marked with asterisks are attributed to complexes of BDP with an impurity.

ve transitions appear coupled to the anharmonic low frequency progression mentioned above.

All vibronic transitions in the droplet spectrum consist of a sharp transition accompa-nied by an asymmetric tail to the blue extending over more than 50 cm⁻¹ . Due to the line shape of the tail and its low relative peak intensity it can only be clearly recognized at the electronic origin and the most intense vibronic transitions e.g. at 322 cm⁻¹ and 763 cm⁻¹ . Enlargements of the electronic origin recorded with different laser intensities are shown in fig. 7.3. At laser intensities lower than used for the solid line spectrum the spectral shape did not alter whereas at higher laser intensities (dotted spectrum) the relative intensities change in favor of the blue tail. Thus, the most intense peak lowest in energy can be assigned to a ZPL while the accompanying features to its blue side, which have no counterpart in the gas phase spectrum, can be attributed to the PW.

(cf. chapter 4) The PW thus consists of sharp features, e.g. at 1.8 cm⁻¹ , followed by a broad asymmetric band to the blue. The laser intensities required to avoid saturation of the ZPL are by at least one order of magnitude lower compared to the anthracene derivatives. (cf. 5) This reflects the much larger absorption cross section or extinction coefficients, respectively, of the PM dyes. [STS⁺90, LABnPMM⁺04, UZH08, SHWJ09]

Fig. 7.3: Excitation spectrum of BDP in helium droplets recorded with different laser inten-sities.(see text)ν₀= 20504 cm⁻¹ .

The line shape of the ZPL at the electronic origin in fig. 7.3 (b) reveals a triple peak structure, which is not fully resolved. This was also observed at the electronic origin of BDP in the supersonic jet. [SF09] BDP contains a boron-atom with a natural abundance ratio of the isotopes ¹¹B and ¹⁰B of 4:1. Differences in the zero-point energy of the two resulting isotopomers¹¹BDP and¹⁰BDP, respectively, in S₀and S₁may lead to a isotopic spectral shift at the electronic origin. Indeed, as shown in fig. 7.4, the line shape at the electronic origin (fig. 7.4 (a)) can be separated into two identical subspectra with the intensity ratio of 4:1 (fig. 7.4 (b)) by applying the maximum entropy method [Fou88].

Each of the subspectra reveal a rotational contour similar as in the gas phase though with a smaller spectral width as is typical for helium droplets. [SF09]

The isotopic spectral shift amounts to 0.81 cm⁻¹ which is in agreement with the values of 0.85 cm⁻¹ at the electronic origin and of 0.92 cm⁻¹ at 21.5 cm⁻¹ excess energy found in the gas phase spectrum. [SF09] Pump-probe experiments in the gas phase revealed that these transitions have contributions originating from different ground states which confirms the assignment to different isotopomers. [SF09] In both the gas phase and the helium droplet spectrum the transition of the lighter isotopomer ¹⁰BDP is shifted to the blue compared to the transition of the heavier isotopomer¹¹BDP indicating an increase of vibrational frequencies upon excitation.

Fig. 7.4: Electronic origin of BDP in helium droplets. The experimental excitation spectrum (a) can be separated using the Maximum Entropy method into the spectra of the two isotopomers (b). The best fit (red) and residuals (green) are also shown in (a).

The feature at 1.8 cm⁻¹in the spectrum of¹¹BDP in fig. 7.4 (b) has no counterpart in the gas phase spectrum. In accordance with the saturation behavior displayed in fig. 7.3 (b) this transition can be assigned to a sharp feature of the PW of the ¹¹BDP isotopomer.

Interestingly, the line width of this peak at 1.8 cm⁻¹ is only ≈ 0.4 cm⁻¹ which is half the width of the rotational envelope of the ZPL. This might indicate that the PW is coupled to only one rotational branch, e.g. the Q-branch.

Larger isotopic shifts than at the electronic origin are expected for vibronic transitions involving excitations of vibrational modes which depend on the nuclear coordinate of the B-atom. The vibronic transitions at 257.9 cm⁻¹ and 259.3 cm⁻¹ , 424.7 cm⁻¹ and 425.6 cm⁻¹, as well as 860.0 cm⁻¹and 861.2 cm⁻¹, respectively, display an intensity ratio of 4:1 as observed at the origin however separated by 1.4 cm⁻¹ , 0.9 cm⁻¹ , and 1.2 cm⁻¹ , respectively. The vibronic transitions at 592.9 cm⁻¹ and 597.4 cm⁻¹ are separated by 4.5 cm⁻¹ and appear with the same intensity ratio of 4:1. Pump-probe experiments or emission spectra recorded upon selective excitation could provide further evidence for the assignments to the two different isotopomers.