Laser spectroscopy of molecular ions: TheÖX̃ transition of the acetylene radical cation

Ultrafast dynamics of photoionized acetylene

Acetylene cations [HCCH](+) produced in the A(2)Σ(g)(+) state by extreme ultraviolet (XUV) photoionization are investigated theoretically, based on a mixed quantum-classical approach. We show that the decay of the A(2)Σ(g)(+) state occurs via both ultrafast isomerization and nonradiative electronic relaxation. We find a time scale for hydrogen migration and electronic decay of about 60 fs, in good agreement with recent XUV-pump/XUV-probe time-resolved experiments on the same system [Phys. Rev. Lett. 105, 263002 (2010)]. Moreover, we predict an efficient vibrational energy redistribution mechanism that quickly transfers excess energy from the isomerization coordinates to slower modes in a few hundred femtoseconds, leading to a partial regeneration of acetylenelike conformations.

Ultrafast extreme ultraviolet induced isomerization of acetylene cations

Ultrafast isomerization of acetylene cations ([HC=CH](+)) in the low-lying excited A(2)Σ(g)(+) state, populated by the absorption of extreme ultraviolet (XUV) photons (38 eV), has been observed at the Free Electron Laser in Hamburg, (FLASH). Recording coincident fragments C(+) + CH2(+) as a function of time between XUV-pump and -probe pulses, generated by a split-mirror device, we find an isomerization time of 52±15 fs in a kinetic energy release (KER) window of 5.8<KER<8 eV, providing clear evidence for the existence of a fast, nonradiative decay channel.

Quartet states of the acetylene cation: electronic structure calculations and spin-orbit coupling terms

Highly correlated ab initio methods have been used to generate one-dimensional cuts of the six-dimensional potential energy surfaces of the quartet and lowest doublet states for the HCCH(+) ion along the CH, CC, and cis and trans bending coordinates. Transition dipole moments and spin-orbit matrix elements are deduced. For the lowest 1 (4)Sigma(u) (+) state, the calculations predict a possible photon emission through the 1 (4)Pi(g)<--1 (4)Sigma(u) (+) transition competing with internal conversion and predissociation processes. The potential surfaces are used together with spin-orbit matrix elements to discuss the metastability and the predissociation processes forming the C(2)HC(2)H(+)+H(+)H products. Multistep spin-orbit induced predissociation pathways are suggested.

The bending vibrational levels of the acetylene cation: A case study of the Renner-Teller effect in a molecule with two degenerate bending vibrations

Forty three vibronic levels of C2H2+, X 2Pi u, with upsilon4 = 0-6, upsilon5 = 0-3, and K = 0-4, lying at energies of 0-3520 cm(-1) above the zero-point level, have been recorded at rotational resolution. These levels were observed by double resonance, using 1+1' two-color pulsed-field ionization zero-kinetic-energy photoelectron spectroscopy. The intermediate states were single rovibrational levels chosen from the A1Au, 4nu3 (K = 1-2), 5nu3 (K = 1), nu2+4nu3 (K = 0), and 47,206 cm(-1) (K = 1) levels of C2H2. Seven of the trans-bending levels of C2H2+ (upsilon4 = 0-3, K = 0-2) had been reported previously by Pratt et al. [J. Chem. Phys. 99, 6233 (1993)]; our results for these levels agree well with theirs. A full analysis has been carried out, including the Renner-Teller effect and the vibrational anharmonicity for both the trans- and cis-bending vibrations. The rotational structure of the lowest 16 vibronic levels (consisting of the complete set of levels with upsilon4 + upsilon5 < or = 2, except for the unobserved upper (2Pi u component of the 2nu4 overtone) could be fitted by least squares using 16 parameters to give an rms deviation of 0.21 cm(-1). The vibronic coupling parameter epsilon5 (about whose magnitude there has been controversy) was determined to be -0.0273(7). For the higher vibronic levels, an additional parameter, r45, was needed to allow for the Darling-Dennison resonance between the two bending manifolds. Almost all the observed levels of the upsilon4 + upsilon5 = 3 and 4 polyads (about half of the predicted number) could then be assigned. In a final fit to 39 vibronic levels with upsilon4 + upsilon5 < or = 5, an rms deviation of 0.34 cm(-1) was obtained using 20 parameters. An interesting finding is that Hund's spin-coupling cases (a) and (b) both occur in the Sigmau components of the nu4 + 2nu5 combination level. The ionization potential of C2H2 (from the lowest rotational level of the ground state to the lowest rotational level of the cation) is found to be 91,953.77 +/- 0.09 cm(-1) (3sigma).

Vinylidene-acetylene cation isomerization investigated by large scale ab initio calculations

The planar isomerization routes of the vinylidene/acetylene cation in the lowest electronic states are accurately examined for the first time, by using large scale MRCI and CCSDT calculations in a complementary way. They are compared with the similar calculations performed for the neutral ground state isomerization. An accurate value of the adiabatic ionization potential of vinylidene (11.26 eV) is predicted. The vinylidene cation lowest state, 1 2A1, follows an almost flat pathway with a shallow secondary minimum on the 1 2A' potential energy surface, before suddenly dropping to the stable acetylene cation ground state, X 2Piu. It is therefore confirmed to be completely unstable with respect to isomerization. The first excited state of the vinylidene cation, 1 2B1, which also correlates with the 2Piu ground state of acetylene cation along a 2A' isomerization route, has been studied at the same level of calculation. This 1 2B1 state is lying only 0.15 eV above the 1 2A1 state, and exhibits a potential energy barrier of 0.55 eV which explains the earlier assignment of this symmetry to the ground state of vinylidene cation. In addition to large scale calculations, a comprehensive description of the important steps of isomerization drawn from a very simple model involving monoconfigurational states is presented. In particular, the behavior of one unique orbital, namely, the 5a1 outer molecular orbital, is shown to completely govern the molecular geometry and energy evolution along the isomerization route of the ground state cation C2H2+.

Femtosecond Dynamics after Ionization: 2-Phenylethyl-N,N-dimethylamine as a Model System for Nonresonant Downhill Charge Transfer in Peptides

The cation of 2-phenylethyl-N,N-dimethylamine (PENNA) offers two local sites for the charge: the amine group and 0.7 eV higher in energy the phenyl chromophore. In this paper, we investigate the dynamics of the charge transfer (CT) from the phenyl to the amine site. We present a femtosecond resonant two-color photoionization spectrum which shows that the femtosecond pump laser pulse is resonant in the phenyl chromophore. As shown previously with resonant wavelengths the aromatic phenyl chromophore can be then selectively ionized. Because the state "charge in the phenyl chromophore" is the first excited state in the PENNA cation, it can relax to the lower-energetic state "charge in the amine site". To follow this CT dynamics, femtosecond probe photoabsorption of green light (vis) is used. The vis light is absorbed by the charged phenyl chromophore, but not by the neutral phenyl and the neutral or cationic amine group. Thus, the absorption of vis photons of the probe laser pulse is switched off by the CT process. For detection of the resonant absorption of two or more vis photons in the cation the intensity of a fragmentation channel is monitored which opens only at high internal energy. The CT dynamics in PENNA cations has a time constant of 80 +/- 28 fs and is therefore not a purely electronic process. Because of its structural similarity to phenylalanine, PENNA is a model system for a downhill charge transfer in peptide cations.

Imaging the Mode-Selected Predissociation of OCS+[(v1v2v3)B̃2Σ+]

A new experimental approach to explore the mode-selected chemistry is proposed and demonstrated here. In this approach a double-resonance excitation scheme is exploited to prepare a well-defined mode or state of a parent ion. A time-sliced velocity imaging technique interrogates the fragment ion from the subsequent predissociation. Application to the title process reveals remarkable mode-specific behaviors despite the long dissociation time associated with the indirect bond-breaking process. A qualitative interpretation of the major findings is surmised.

The electronic spectrum of protonated adenine: Theory and experiment

In this work we present the results of a combined experimental and theoretical study concerned with the question how a proton changes the electronic spectrum and dynamics of adenine. In the experimental part, isolated adenine ions have been formed by electro-spray ionisation, stored, mass-selected and cooled in a Paul trap and dissociated by resonant photoexcitation with ns UV laser pulses. The S(0)-S1 spectrum of protonated adenine recorded by fragment ion detection lies in a similar energy range as the first pipi* transition of neutral 9H-adenine. It shows a flat onset with a broad substructure, indicating a large S(0)-S1 geometry shift and an ultra-short lifetime. In the theoretical part, relative energies of the ground and the excited states of the most important tautomers have been calculated by means of a combined density functional theory and multi-reference configuration interaction approach. Protonation at the nitrogen in position 1 of the neutral 9H-adenine tautomer yields the most stable protonated adenine species, 1H-9H-A+. The 3H-7H-A+ and the 3H-9H-A+ tautomers, formed by protonation of 7H- and 9H-adenine in 3-position, are higher in energy by 162 cm(-1) and 688 cm(-1), respectively. Other tautomers lie at considerably higher energies. Calculated vertical absorption spectra are reported for all investigated tautomers whereas geometry optimisations of excited states have been carried out only for the most interesting ones. The S1 state energies and geometries are found to depend on the protonation site. The theoretical data match best with the experimental onset of the spectrum for the 1H-9H-A+ tautomer although we cannot definitely exclude contributions to the experimental spectrum from the 3H-7H-A+ tautomer at higher energies. The vertical S(0)--> S1 excitation energy is similar to the one in neutral 9H-adenine. As for the neutral adenine, we find a conical intersection of the S1 of protonated adenine with the ground state in an out-of-plane coordinate but at lower energies and accessible without barrier.