Energy level diagram indicating the resonant ionization scheme used to record IR spectra of ions (left). Timing diagram indicating temporal structure and timing of the ionization (UV) and excitation (IR) laser pulses (upper right). Schematic time-of-flight spectrum indicating the contributions of bare ions produced by the UV ionization of neutral molecules and those resulting from the IR excitation of ions in helium droplets.
(Upper panel) IR excitation spectrum of aniline ions in helium droplets consisting on average of 2700 atoms. (Lower panel) Expanded view of the same spectrum. The peaks denoted by an asterisk correspond to transitions of neutral aniline molecules in helium droplets, while the peak denoted by a diamond corresponds to the aniline2 + complex.
IR excitation spectrum of neutral aniline molecules in helium droplets. The transition denoted by an asterisk belongs to the aniline cation.
IR excitation spectrum of aniline2 + and aniline3 + complexes in helium droplets and the calculated spectrum of aniline2 + for the NH-N, NH-π, and head-to-tail geometries, see text for details.
Structures of the aniline2 + cation calculated by DFT at the B3LYP/6-311++G(df,pd) level of theory.
IR excitation spectrum of the styrene cation in helium droplets (upper panel) and the theoretical spectrum based on DFT calculations at the B3LYP\6-311++G(df,pd) level of theory (lower panel).
(Upper panel) IR excitation spectrum of 1,1-diphenylethylene cations in helium droplets. (Middle panel) The same spectrum recorded with a factor two reduced IR intensity. (Lower panel) Theoretical spectrum of the 1,1-diphenylethylene radical cation based on DFT B3LYP\6-311++G(df,pd) calculations. The intensity of the strongest transition at 1086 cm−1, denoted by an asterisk, has been reduced by a factor 4 for a better comparison with the experimental spectra.
(Upper panel) Time-of-flight mass spectra in the absence and presence of IR radiation resonant with the transition at 1531 cm−1 of styrene cations in helium droplets. (Lower panel) Time-of-flight spectra of styrene cations in the presence and absence of resonant IR radiation. The inset shows the difference between the two time-of-flight spectra.
Ion images and corresponding speed distribution of desolvated 1,1-diphenylethylene cations following excitation at 1547 and 335 cm−1. The solid line is a fit of the data to a Maxwell-Boltzmann distribution with a temperature as indicated in the figure.
Translational temperature of aniline, styrene, and 1,1-diphenylethylene (DPE) cations ejected from helium droplets as function of excitation energy. The droplets consist on average of 2700 helium atoms.
Comparison of the vibrational transition frequencies (cm−1) of the aniline radical cation.
Comparison of the vibrational transition frequencies (cm−1) of the aniline molecule.
Comparison of the observed vibrational transition frequencies of styrene radical cation and the scaled theoretical harmonic frequencies (cm−1) calculated using DFT at the B3LYP\6-311++G(df,pd) level of theory. Also indicated are the calculated intensities (km/mol).
Comparison of the observed vibrational transition frequencies (cm−1) of neutral styrene in helium droplets and in the gas phase.
Comparison of the experimental vibrational transition frequencies of the 1,1-diphenylethylene radical cation in helium droplets with the scaled harmonic frequencies (cm−1) calculated using DFT at the B3LYP\6-311++G(df,pd) level of theory. Also indicated are the calculated transition intensities (km/mol).
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