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Abstract:
Both vibrationally and rotationally resolved spectra of the S1←S0 transition in jet-cooled triphenylamine (TPA) around 340-320 nm are reported. Medium resolution spectra (0.5–1.0 cm−1 resolution) are recorded using (1 + 1)-resonance enhanced multi photon ionization (REMPI) with mass selective time-of-flight (TOF) detection in a pulsed molecular beam apparatus. The origin of the S1←S0 transition is at 29520.7 cm−1, higher than halfway to the ionization potential (IP) found at 6.89 eV. A vibratioal progression in the symmetric torsion mode (114 cm−1) as well as in the symmetric C-N stretching mode (280 cm−1) is observed in the electronic spectra. The spectrum of the most abundant isomer of the TPA-Ar (TPA-Kr) complexes is blue-shifted by 211 cm−1 (216 cm−1) with respect to the spectrum of the free TPA molecule. High-resolution (the resolution mainly being determined by the natural linewidth of the transition, i.e. 36 MHz) spectra are recorded using laser induced fluorescence (LIF) in a cw molecular beam apparatus. Individual rotational transitions are resolved and the spectrum shows unambiguously that TPA is a symmetric top molecule. The rotational constant B″ in the S0 state of TPA is equal to B″ = 403.7 ± 0.5 MHz. Upon S1←S0 excitation both B and C increase with 7.4 ± 0.1 MHz and 2.8 ± 0.1 MHz, respectively. The spectrum of the blue-shifted TPA-Ar isomer is the spectrum of a symmetric top molecule as well, and therefore the Ar atom has to be located on the C3 symmetry axis, either on top of or underneath the umbrella formed by the phenyl rings. It appears that when Ar or Kr forms a complex with TPA, the first Ar, Kr, atom goes preferentially in a position on the C3 symmetry axis of TPA, a position which causes an abnormal blue-shift of the spectrum. With the first rare-gas atom located in this special position, the second rare-gas atom is forced into a “normal” position, i.e. above one of the phenyl rings, causing a normal red-shift with respect to the TPA-Ar complex.
