D. Gauyacq

s and d Rydberg series of NO probed by double resonance multiphoton ionization

The s and d Rydberg states of NO with v = 0 have been observed for 6 leqslant; n* leqslant; 40 by double resonance multiphoton ionization which was three-photon resonant with the 3pπC 2∏ (v = 0) Rydberg intermediate state. Strong parity selection is shown to occur in the intermediate level despite the fact that the Λ-doublets are not resolved. The OODR spectra are thereby considerably simplified. A unified description of the rotational-electronic structure of the s and d Rydberg states in terms of quantum defect theory is presented, extending from the low n* s, d supercomplexes up beyond the ionization threshold. Electronic radial dipole transition moments are calculated and included in the MQDT analysis, giving evidence for a Cooper minimum in the region of the supercomplex n* = 5. For 25 leqslant; n* leqslant; 40 fringes are observed in the OODR spectrum and are interpreted in terms of rotational l-uncoupling, electronic s ∼d mixing and energy dependent electronic dipole transition moments. The fringe f...

A,C, and D electronic states of the Ar–NO van der Waals molecule revisited: Experiment and theory

The A–X transition of ArNO has been reinvestigated by laser induced fluorescence (LIF) both in the bound–free and bound–bound region. The discrete part of the spectrum is at least two orders of magnitude weaker than the continuum part, indicative of a large change in geometry from the ground state. This very different configuration, both from the ground state and from the C and D states, can only be explained by strong interactions, induced by the perturbing argon atom, between the excited states of the van der Waals complex converging to the 3sσ,A, 3pπ,C, and 3pσ,D Rydberg states of NO. In order to quantitatively understand the observed structure of the A–X, C–X, and D–X excitation spectra, a global theoretical approach is proposed, based on ab initio calculations of the potential energy surfaces in the planar A′ and A″ symmetries, including a configuration interaction between the states of same symmetry. Small adjustments of the diabatic energy surfaces lead to a satisfactory agreement between the obser...

nf Rydberg complexes of NO in a magnetic field, probed by double resonance multiphoton ionization

nf (v=1) Rydberg states of NO have been probed by double resonance multiphoton ionization in a 1 T external magnetic field. Due to the nonpenetrating character of the f orbitals, these Rydberg states are very sensitive probes of any external perturbation. As n increases, a decoupling of the angular momentum l of the Rydberg electron from the molecular frame occurs gradually, as the magnetic interaction becomes more and more important with respect to intramolecular forces. Up to n≂15, only the linear Zeeman perturbation has been taken into account. The rotational–electronic structure of the 7f and 15f states has been interpreted theoretically by considering the linear Zeeman perturbation in addition to the Coulombic interaction and the long range interaction due to the quadrupole moment and the polarizability of the ion core. The intensities and line positions of the transitions from the intermediate Au20092Σ+,v=1 level to the 7f and 15f levels have been calculated. The alignment of the N, MS, MN Zeeman sublev...

s and d Rydberg complexes of NO probed by double-resonance multiphoton ionisation in the region n* = 5 to n* = 25; multichannel quantum defect analysis. Part II

Optical-optical double-resonance multiphoton ionisation spectroscopy is used to probe the n*s and n*d Rydberg series converging to the first rotational levels of NO+X1Σ+, v = 0. Intermediate values of the principal quantum number probed are 5 ⩽ n* ⩽ 2>5. The initial excitation occurs via one of two rotational-parity levels of the C 2Π state (N′ = 2 and N′ = 6). A description of the rotational-electronic structure of the observed supercomplexes is proposed in terms of multichannel quantum defect theory (MQDT). The observation of ‘forbidden’ n*f levels is tentatively explained both in terms of d-f mixing in the upper levels and of a d-wave contribution to the 3pπ orbital of the C state. Rydberg-valence interactions show up as predissociation, which strongly competes with ionisation in the intermediate n*d supercomplexes with 8 ⩽ n* ⩽ 1>2. This work complements the previous analysis of the s and d Rydberg states of NO corresponding to n* ⩽ 8> and 25 ⩽ n* ⩽ 4>0 [15].

Visible emission from the vibrationally hot C2H radical following vacuum-ultraviolet photolysis of acetylene: Experiment and theory

Photolysis of acetylene has been performed by vacuum-ultraviolet excitation with the synchrotron radiation via the Rydberg states converging to the first ionization potential (IP) at 11.4 eV. Only the visible fluorescence of the ethynyl radical was observed in the Au200a2Π–Xu200a2Σ+ system. Excitation of several Rydberg states of acetylene over a large energy range between 9 and 11.4 eV allowed us to observe for the first time the evolution of this continuum with increasing Rydberg excitation. Intensity calculations based on accurate ab initio potential energy surfaces of C2H were performed by using a one-dimensional model accounting for the large-amplitude motion of the H atom around the C–C bond and for the overall rotation of the radical. These calculations successfully reproduce the observed visible continuum (maximum at 500 nm and blue side cutoff at 400 nm) and bring new information on the distribution of the internal energy deposited in the fragment. For most excited Rydberg states, predissociation occurs...

Observation of the lowest 1Δu Rydberg state of acetylene by multiphoton ionization spectroscopy

The 1Δu, 3dπ component of the lowest ungerade 3d+4s Rydberg supercomplex of acetylene has been observed by (3+1) photon ionization spectroscopy of C2H2 and C2D2. In both isotopic species the vibrationless level of this new electronic state is sufficiently long lived to support rotational structure. From the observed isotopic shift and from the rotational and vibrational parameters determined in the present analyses, this state could be unambiguously identified as the lowest Rydberg state of 1Δu symmetry, belonging to the 3d complex of acetylene. The weak intensity of this three‐photon 1Δu–1Σ+g electronic transition is discussed in comparison with the other Rydberg transitions within the same supercomplex. Excited vibronic bands in the same energy region allowed to determine the ν1 and ν2 frequencies for some components of this Rydberg supercomplex. The absence of the nearby H 1Πu, 3dδ Rydberg state in the MPI spectra is discussed in terms of predissociation.

Detection of NO traces Using resonantly enhanced multiphoton ionization: A method for monitoring atmospheric pollutants

Resonantly enhanced multiphoton ionization (REMPI) is proposed as an ultrasensitive detection method forreal-time monitoring of atmospheric pollutants in situ. The technique is demonstrated in the laboratory for NO diluted in pure nitrogen at 560 mbar. The MPI current resulting from (2+1) photon ionization of NO via the resonantC2H (v=0) state has been measured for several NO concentrations. Detection levels as low as 50 ppt have been obtained.

Vacuum–ultraviolet photodissociation of C2H2 via Rydberg states: a study of the fluorescent pathways

Abstract The visible fluorescence produced by vacuum–ultraviolet photodissociation of acetylene, particularly through the Rydberg states, has been studied in a gas-flow system using synchrotron radiation as a light source between 154 and 60 nm and an original light collection device. Excitation of the Rydberg states below the first IP reveals only the C2H product by its A–X continuum emission spectrum between 330 and 900 nm. Evolution in the emission spectral profile as a function of excitation wavelength has been observed, indicative of specific internal distribution favoring the bending levels of the A state of C2H. A new assignment of the Rydberg series converging to the first excited state A 2 A g of the cation is proposed on the basis of recent experimental and theoretical characterization of this state and leads to an approximate value of the C–C stretching frequency of the cation (ν2≈1500 cm−1).

Spectroscopy and dynamics of the Rydberg states of C2H2 and their relevance to astrophysical photochemistry

In order to understand the role of the Rydberg states in the photodestruction of acetylene in circumstellar envelopes, we have investigated the spectroscopy of some of the ungerade and gerade states by resonant multiphoton ionization in a magnetic bottle photoelectron spectrometer. The ungerade ns + nd Rydberg supercomplexes of C2H2 have been investigated by one–colour (3+1) multiphoton ionization, in the energy region 79,500–87,000 cm−1. Our spectral data show a very different behaviour of the various components of these supercomplexes towards predissociation. The highly excited gerade nf Rydberg states have been observed for the first time, by two–colour (3+1′) multiphoton ionization. A value for the adiabatic ionization potential, 91,956 cm−1, has been obtained from the observed series limit and from earlier high–resolution infrared data on the ground state of the ion. This value is 4 cm−1 higher than the one measured by ZEKE spectroscopy. The apparent broadening of the observed Rydberg features may originate both from Zeeman ‘broadening’ of the non–penetrating nf complexes and from predissociation. The np Rydberg states, for which observation is fully allowed in the same spectral region, do not appear in the ionization signal at least in the low–n range, most probably because of their very short lifetime. In the high–energy region, spectral congestion, probably involving p–f mixing, occurs.

Zeeman effect in the two-photon excitation spectrum of the C 2Π state of NO

Abstract The two-photon resonant, three-photon ionization spectrum of NO via the resonant C 2 Π, ν = 0 state has been recorded in an external magnetic field of 0.93 T. The Zeeman effect in the upper C 2 Π and the lower X 2 Π states has been taken into account by diagonalizing an energy matrix in the case (b) basis set for both 2 Π states. The intensities of the observed rotational lines have been interpreted in terms of propensity rules for the resonant two-photon transition via the quasi-resonant intermediate state A 2 Σ + . The relative population of the Zeeman sublevels has been calculated in order to show how the optical excitation in an external magnetic field results into an orientation of the rotational levels of the upper state, even in the case of a linearly polarized laser excitation.