The study of the S1(1A1)<--S0(1A1) transition of benzo[g,h,i]perylene (BghiP, C22H12) in supersonic jets and solid rare gas matrices is reported. In the jet-cooled spectrum, the origin band position is located at 25,027.1+/-0.2 cm-1, the assignment being supported by the analysis of vibrational shifts and rotational band contours. Except for the origin band, which is weak, all bands are attributed to the fundamental excitation of nontotally symmetric b1 vibrational modes of S1. The intensity pattern is interpreted as a consequence of the weak oscillator strength of the electronic transition combined with intensity-borrowing through vibronic interaction between the S1(1A1) and S2(1B1) states. The spectra of the S1(1A1)<--S0(1A1) and S2(1B1)<--S0(1A1) transitions have also been measured for BghiP in solid neon and argon matrices. The comparison of the redshifts determined for either transition reveals that the polarizability of BghiP is larger in its S2 than in its S1 state. Bandwidths of 2.7 cm-1 measured in supersonic jets, which provide conditions relevant for astrophysics, are similar to those of most diffuse interstellar bands. The electronic transitions of BghiP are found to lie outside the ranges covered by present databases. From the comparison between experimental spectra and theoretical computations, it is concluded that the accuracy of empirical and ab initio approaches in predicting electronic energies is still not sufficient to identify astrophysically interesting candidates for spectroscopic laboratory studies.

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The jet-cooled ultraviolet direct absorption spectrum of the amino acid tryptophan is reported. The spectrum measured by cavity ring-down laser absorption spectroscopy covers the region where the origin bands of the S(1) <-- S(0) transitions of six conformers (A to F) are located. Tryptophan was transferred into the gas phase by two different methods, namely, thermal heating and laser vaporization. The latter technique allowed us to obtain higher densities of tryptophan in the jet at the time when it was probed for spectroscopy. It also avoided thermal decomposition of the sample. On the other hand, a higher signal-to-noise ratio was obtained with thermal heating. Measurements were carried out by laser-induced fluorescence as well. The comparison of the absorption and excitation spectra reveals a higher fluorescence yield and a shorter radiative lifetime for the S(1) state of conformer A relative to the other conformers. Moreover, the comparison of our spectra with each other and with literature data led us to assign a band to a new conformer, which we named G. Finally, the theoretical structure and vibrational frequencies obtained from density functional theory based calculations confirm that the progression observed in the S(1) <-- S(0) spectrum of conformer A is consistent with a torsional motion of the amino acid side chain relative to the indole chromophore.

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