Anharmonic Progressions Research Articles

Torsional vibrational structure of the propene radical cation studied by high-resolution photoelectron spectroscopy

The pulsed-field-ionization zero-kinetic-energy photoelectron spectra of the X̃(+2)A(")←X̃(1)A' transition of CH(3)CHCH(2) (propene), CD(3)CDCD(2), and several partially deuterated isotopomers have been recorded in the region of their adiabatic ionization thresholds and up to 2000 cm(-1) of internal energy of the cations. The vibrational structure has been assigned on the basis of the frequency shifts resulting from deuteration of selected sites of the propene molecule. Two highly anharmonic progressions have been identified and assigned to the two torsional modes of the propene cation, the methyl and methylene torsions. The positions of the torsional levels could be approximately reproduced using one-dimensional models, allowing a semi-quantitative description of the potential energy surface along each torsional coordinate. The observation of forbidden vibrational bands and the analysis of their partially resolved rotational contours reveal the importance of the vibronic coupling between the X̃(+2)A(") and the Ã(+2)A(') states mediated by the methylene (ν(20)) and methyl (ν(21)) torsional modes.

Pulse-train control of multiphoton transitions in anharmonic progressions: Resonance loci and resonance ridges

The properties of pulse-train induced multiphoton excitation in anharmonic progressions and the accumulation of population in a specific rung state are investigated by means of numerical simulations. It is shown how and under which conditions resonant π-pulses and multiple-π pulses can be split into trains of fractional π-pulses driving the same transition. Standardized train forms are considered with sub-pulses of equal (gaussian) shapes and equal, but tunable pulse-to-pulse delays and pulse-to-pulse phase shifts. The increased number of tuning parameters together with the handle on the number of sub-pulses gives rise to a remarkable variability in the control of state-specific population transfer, where simple zero-order estimates assist the determination of the parameters. Each π- or multiple-π pulse is replaced by a resonance locus in parameter space representing an infinite set of π-trains. The loci span extended frequency ranges that increase with increasing sub-pulse number. Their projection onto the frequency-field strength plane gives rise to elliptically shaped closed curves, termed resonance ridges, which replace the singular points mapped out by simple π- and multiple-π pulses. In the subspace of pulse-to-pulse delays and pulse-to-pulse phase shifts the resonance loci are characterized by phase recurrence relations, whose number and complexity increases with increasing numbers of sub-pulses. Our results indicate that pulse trains may be a powerful tool for the control of parallel or branching multiphoton transitions and for the elimination of background and intruder state population.

Phase-sensitive stimulated Raman adiabatic passage in dipolar extended lambda systems

The authors investigate the possible phase-sensitive behavior of (multiphoton) stimulated Raman adiabatic passage population transfer in extended lambda systems, if more than one state of an anharmonic progression of target levels is accessible in transitions of different photonicities. They use a minimal model four-level system (4LS) with one initial state separated from two target states by an apex state. The parameters of the 4LS are adapted from the bend states of the HCN-HNC system. Using a dressed-state analysis within the rotating wave approximation (RWA), the authors identify phase-dependent diabatic transitions between the two dressed states contributing to the state vector as the mechanism leading to phase-sensitive target populations. The essential features giving rise to the phase dependence are found to be different (non-zero-) diagonal elements of the dipole matrix, i.e., permanent dipole moments, and the presence of a direct two-photon overtone coupling between the apex state and the lower target state which formally enters the RWA Hamiltonian upon inclusion of permanent dipole moments. Among the parameters controlling the extent of the effect are the anharmonic properties of the target progression and the absolute values of the field frequencies, so that in view of the requirement to tune the driving fields into the vicinity of resonance, details of the level structure are of importance. A comparative numerical study executed without invoking RWA shows that qualitatively there are similar trends in the appearance of phase sensitivity, although the effects are considerably more pronounced in the full treatment. In the full treatment the authors also explore off-resonance conditions and discuss the signatures of phase sensitivity in the target populations.

Excited state hydrogen transfer in fluorophenol·ammonia clusters studied by two-color REMPI spectroscopy

Two-color (1 + 1') REMPI mass spectra of o-, m- and p-fluorophenol.ammonia (1 ration) clusters were measured with a long delay time between excitation and ionization lasers. The appearance of NH(4)(NH(3))(n-1)(+) with 100 ns delay after exciting the S(1) state is a strong indication of generation of long-lived species via S(1). In analogy with the phenol.ammonia clusters, we conclude that an excited state hydrogen transfer reaction occurs in o-, m- and p-fluorophenol.ammonia clusters. The S(1)-S(0) transition of o-, m- and p-fluorophenol.ammonia (1 : 1) clusters were measured by the (1 + 1') REMPI spectra, while larger (1 ration) cluster (n = 2-4) were observed by monitoring the long-lived NH(4)(NH(3))(n-1) clusters action spectra. The vibronic structures of m- and p-fluorophenol.ammonia clusters are assigned based on vibrational calculations in S(0). The o-fluorophenol.ammonia (1 : 1) cluster shows an anharmonic progression that is analyzed by a one-dimensional internal rotational motion of the ammonia molecule. The interaction between the ammonia molecule and the fluorine atom, and its change upon electronic excitation are suggested. The broad action spectra observed for the o-fluorophenol.ammonia (1 : n) cluster (n>== 2) suggest the excited state hydrogen transfer is faster than in m- and p-fluorophenol.ammonia clusters. The different reaction rates between o-, m- and p-fluorophenol.ammonia clusters are found from comparison between the REMPI and action spectra.

An accurate ab initio quartic force field and vibrational frequencies for cyclopropenylidene

The singles and doubles coupled-cluster method that includes a perturbative correction for connected triple excitations, denoted CCSD(T), is used in conjunction with an spdf quality one-particle basis set to determine an accurate quartic force field for cyclopropenylidene. A second-order perturbation theory treatment of vibrational anharmonicities, together with proper treatment of Fermi resonances, is used to predict fundamental vibrational frequencies of cyclopropenylidene and its 13C and deuterium isotopomers. Agreement between theory and the available experimental data is excellent. It is demonstrated that four vibrational bands assigned to cyclopropenylidene in 1984 matrix isolation experiments are correct, contrary to a recent suggestion. The anharmonic progression in the C-H stretches is examined and found to be similar for both the symmetric and antisymmetric C-H stretches, contrary to the findings from another recent study.

(2+1) REMPI spectra of the I2–Krand I2–N2van der Waals complexes and the (2+1[prime ]) ZEKE[ndash ]PFI photoelectron spectrum of I2+–Kr

We present the (2 + 1) mass-resolved resonance-enhanced multiphoton ionisation (REMPI) spectrum of the [2Π3/2]c5d;Ωg (Ω = 0,2) Rydberg states of the I2–Kr van der Waals complex. Anharmonic progressions in the I2···Kr van der Waals stretching mode have been observed with e values of 49 ± 2 and 47 ± 2 cm−1 for the [2Π3/2]c5d;Ω = 0g and [2Π3/2]c5d;Ω = 2g states, respectively. The [2Π3/2]c5d;Ω = 2g state of I2–Kr has been used as an intermediate in a two-colour (2 + 1′) zero kinetic energy pulsed-field ionisation (ZEKE–PFI) photoelectron study of the 2Π3/2,g state of the I2+–Kr ionic complex. The adiabatic ionisation energy was determined to be 74248 ± 3 cm−1. An anharmonic vibrational progression in the I2+···Kr van der Waals stretching mode was observed giving a value of e = 55 ± 2 cm−1 for the 2Π3/2,g state of the complex. The [2Π1/2]c6s;Ω = 1g Rydberg state of the I2–N2 van der Waals complex has also been observed using REMPI spectroscopy. Anharmonic progressions in the I2···N2 van der Waals stretching mode have been observed with e = 71 ± 2 cm−1. Comparisons are made between the spectroscopic data obtained for the Rydberg states of I2–Kr and I2–N2 with the analogous Rydberg states in I2–Ar.

S2−S0 Spectroscopy of the van der Waals Complexes of Azulene with Rare Gases

The S2−S0 fluorescence emission and excitation spectra of jet-cooled azulene and its complexes with the rare gases, Ne, Ar, Kr, and Xe, have been measured. Features due to AZ·RGn (n = 1−4 when RG = Ar, Kr, Xe and n = 1 when RG = Ne) are observed in the S2−S0 fluorescence excitation spectra when azulene is coexpanded with the rare gases. The microscopic solvent shifts, δν, of the origin bands for each complex scale linearly with the polarizability of the adatom(s), indicating that binding is dominated by dispersive interactions. This conclusion has been confirmed in calculations of the separate contributions of dispersion and induction to δν. Mildly anharmonic progressions in very low frequency excited-state intermolecular (van der Waals) modes are attached to the origin bands of each AZ·Krn and AZ·Xen complex. The dominant progression is assigned to single quantum changes in that excited-state bending mode which involves motion of the adatom(s) in the plane perpendicular to the azulene ring containing t...

The spectroscopy of Rydberg excited iodine-argon van der Waals complexes

The Rydberg excited I 2-Ar complex formed in a supersonic free jet has been observed using mass resolved (2+1 ) REMPI spectroscopy. The present work has concentrated on the [ 2Π 3 2 ] g 7s, Ω= 1, 2 Rydberg series of I 2-Ar. The spectrum exhibits anharmonic progressions in the I 2…Ar van der Waals stretching mode for each Rydberg state with ω e values of 59±2 and 53±2 cm −1 for the two states. The corresponding zero-point dissociation energies, calculated from the observed spectral red-shifts were determined to be 568±5 and 504±5 cm −1. All the experimental data indicate that the complex has a T-shaped geometry in both Rydberg states.