Three-photon Ionization Research Articles

Laser photodissociation of C7H7+ created through multiphoton ionization of para-chlorotoluene

The nature of the C7H7+ ion created through resonant dissociative multiphotoionization of para-chlorotoluene by an ultraviolet (UV) laser was investigated thanks to its interaction with a second laser beam. The dissociation pattern corresponding to one or several photon absorption could be observed. Cross section for the one-photon absorption in the 265/530 nm range revealed the presence of the tropylium and benzyl isomers and suggested they possessed substantial internal energy. This was confirmed by the study of the C7H7++hν■C5H5++C2H2 reaction, and more precisely of its rate and of the kinetic energy released. A ladder switch mechanism for the three-photon dissociative ionization of para-chlorotoluene leading to C7H7+ is shown to agree with our results.

Investigation of the 3d Rydberg state of CS 2 by multiphoton ionization

Two-and three-photon resonance-enhanced multiphoton ionization spectra of the 3d Rydberg states of CS 2 in the 154–160 nm region are reported. Noting that the selection rules for multiphoton transitions are quite different from those of one-photon transitions, the electronic origin of the 3dΔ g Rydberg←X transition of CS 2 is redefined to 63323 cm −1. The associated vibrational frequencies of the ν 1, ν 2 and the 2ν 1, 2ν 2 modes and the separation of the Π and Φ components of the ν 2 vibration in this excited state are determined.

Multiphoton ionization of Rb2 in the wavelength range 620-670 nm

The relative production rates of Rb+ and Rb+2 ions through the multiphoton ionization (MPI) of Rb2 and the effective order of non-linearity k for these ions were measured in the wavelength range 620-670 nm. The measured results indicated that these ions were produced by two- or three-photon ionization of Rb2. The Franck-Condon factors calculated for the transition from the ground state (X 1 Sigma +g) to the first intermediate state (B 1 Pi u) were in good agreement with the production rates obtained for Rb+ and Rb+2 ions. From these reasons the authors concluded that in the process of MPI of Rb2 the transition to the Bu 1 Pi u state plays a crucial role in the production of both ions. The competitive relations between the production processes of Rb+2 and Rb+ ions were derived. In the wavelength range at around 669 nm the ratio of the number of Rb+ ions to that of Rb+2 ions produced through three-photon ionization of Rb2 was about 4/1.

Photoelectron spectroscopy of the linear (Ã 2A 1)3pb 21B 2 Rydberg state of water

Photoelectron spectra are reported for two-photon-resonant, three-photon ionization via several vibrational levels of the linear (à 2A 1)3pb 2 1B 2 Rydberg state of H 2O and D 2O. In the dominant photoionization pathway, the ejection of the Rydberg electron occurs with no change in the electronic or vibrational state of the ion core, providing direct confirmation of the vibrational numbering of the 1B 2 state proposed by Abramson et al. Photoionization accompanied by excitation of the symmetric stretch in the resulting ion is also observed, reflecting the difference in bond lengths between the 1B 2 Rydberg state and the à 2A 1 ionic state.

Second-harmonic generation in atomic hydrogen induced by a charge-separation field

Second-harmonic generation in the absence of an externally appplied electric field was investigated near 121.6 nm in atomic hydrogen. It was shown that three-photon ionization produces a local electric field due to charge separation that in turn induces second-harmonic generation in this otherwise isotropic medium. The magnitude of the charge-separation field was determined from the observed second-harmonic power and was found to be in good agreement with a calculated value based on the measured number of photo-ions, the interaction volume, and the incident laser pulse duration.

A simplified method for absolute MPI cross-section measurements. Application to three-photon non-resonant ionization of Xe at 266 nm

A simple method to measure absolute MPI cross sections based on a direct measurement of the ions, produced out of the focal region, is described. The method avoids the problem of the complicated effective-volume evaluation at the focus of a laser beam, difficulties in estimation of the absolute number of ions, as well as the problems arising due to measurements in the saturation regime. The method is best for high cross-section measurements; however, in order to demonstrate its feasibility, it has been applied to the study of non-resonant MPI of Xe. This three-photon ionization cross section, measured for the first time at the fourth harmonic of a mode-locked Nd:YAG laser (266 nm), is found to be (1.1 ± 0.6) × 10 −82 cm 6 s 2.

Theory of multiphoton ionization tested on negative ions.

In a recently published perturbation theory study of multiphoton-ionization cross sections of H{sup {minus}} (S. Geltman, Phys. Rev. A 43, 4930 (1991)), a serious discrepancy was reported between the model-based results and previous {ital ab} {ital initio} Floquet-theorem-based coordinate rotation results (M. Crance, J. Phys. B 23, L285 (1990)). We present two-and three-photon ionization cross sections from many-electron, many-photon-theory nonperturbative calculations, which essentially agree with Geltman's results. This finding supports our earlier conclusions on the physics of photodetachment without or with an external static field.

Generation of harmonic radiation by hydrogen atoms in intense laser fields.

A variational principle for the time-dependent Schr\"odinger equation with an anisotropic Gaussian wave packet as a trial wave function is used to calculate the nonperturbative response of a hydrogen atom to a strong linearly polarized laser field. This method leads to equations of motion for the position expectation value and allows one to study a classical (\ensuremath{\Elzxh}=0) limit as well as the first quantum corrections. These corrections are shown to be responsible for a strong enhancement in the generation of the third and fifth harmonic by the bound-state part of the electronic wave function for field intensities of the order of ${10}^{14}$ W/${\mathrm{cm}}^{2}$ and a laser frequency corresponding to three-photon ionization. For a low-frequency field a strong nonperturbative enhancement of harmonics of orders 9 to 15 is observed. It is concluded that the plateau at even higher harmonics observed recently in experiment and in numerical calculations is caused by the ionizing parts of the wave function that are not represented in the present calculation.

Stabilization of a model atom undergoing three-photon ionization.

We present evidence obtained from numerical experiments that strong-field stabilization of atoms can occur in the context of photon frequencies as low as those corresponding to three-photon ionization of the ground state of a quasihydrogenic atom.

Atomic quasicontinuum effect on spectra of doubly resonant ionisation

Abstract A model of doubly resonant three-photon ionisation is considered with the inclusion of the quasicontinuum of excited levels lying below the ionisation threshold. A profound effect of the type of quasicontinuum, its level density and spacing on both the photoelectron energy and the ionisation spectra is analysed in different time and intensity limits.

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

The two-photon resonant, three-photon ionization spectrum of NO via the resonant C 2Π, ν = 0 state has been recorded in an external magnetic field of 0.93 T. The Zeeman effect in the upper C 2Π and the lower X 2Π states has been taken into account by diagonalizing an energy matrix in the case (b) basis set for both 2Π 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 2Σ +. 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.

Spectroscopy and photodissociation of Rydberg states of N2O

The optical spectrum of N2O in the region from 80 000 to 87 000 cm−1 has been investigated by two- and three-photon resonant multiphoton ionization mass spectrometry. State symmetry identification has been performed by the use of linearly and circularly polarized light in both two- and three-photon absorption, conclusively confirming the assignment of the spectrum as that of the 3pσ 1Π Rydberg state of N2O. The new spectra and polarization experiments also permit observation and analysis of previously unreported vibronic coupling effects in this state. Mass spectral data reveal significant variation in the degrees of fragmentation into NO+ and N+2 as a function of vibrational and rotational excitation in the intermediate state. An apparent progression of a previously unreported second electronic band is also observed and for which a possible assignment is discussed.

The role of spontaneous relaxation in the three-photon ionization of Ba through the two-photon resonant 5d7s 3D2 state

The influence of spontaneous relaxation on the multiphoton ionization process is considered. Experimental and theoretical investigations have shown that with a comparable weak field there is an interval of low stability of the ionization process, when the probabilities of the competing processes of spontaneous relaxation and ionization are nearly equal. It leads to the unusually rapid growth of the amplitude of the discrete two-photon 5d7s 3D2 resonance in the yield of Ba+, when the field strength was increased very little.

Observation of an electronic state of C2H near 9 eV by resonance ionization spectroscopy

A new electronic state of C2H, tentatively assigned as the 3pσ 2Π Rydberg state, has been observed by resonance-enhanced multiphoton spectroscopy (REMPI). The observed absorptions originate from vibrationally excited C2H and C2D formed in the photodissociation of C2H2, C2D2, and C2HD at 193 nm in a molecular beam. Two-laser experiments and REMPI spectroscopy of photofragments of the dissociation of C2HD were used to assign the carriers of the observed spectra to C2H and C2D. Two-photon resonant, three-photon ionization (2+1 REMPI) of C2H and C2D is accompanied by substantial fragmentation to C+2 ions caused by multiple photon absorption by resonantly excited C2H and C2D. The identity of the lower C2H electronic state(s) (Ã 2Π or X̃ 2Σ+ ) was not established.

Polarization dependence of direct three-photon ionization for Ca, Sr and Ba atoms

Experimental results for the probability ratio R3 during three-photon ionization of Ca, Sr and Ba atoms in circularly and linearly polarized fields are presented for the principal quantum number range n=4-13. Unlike the alkali atoms, in most cases the measured R3 magnitudes are considerably less than the values estimated by the Klarsfield-Maquet factorial formula.

Multiphoton dissociation of some aromatic molecules. Formation of atomic carbon in the 2p21S0 excited state

Carbon atoms in the high energy metastable state 2p2 1S0 have been detected for the first time in multiphoton dissociation of some aromatic molecules, using tunable laser light in the region of 380 to 393 nm. The detection is based on the two-photon resonances2p 1S0 → → 3p1D2 and 2p1S0 → → 3p1S0 three-photon ionization of atomic carbon, following the dissociation of the molecule, during the same laser pulse. The results are of particular interest in the field of nonlinear photochemistry.

Second-harmonic generation at Lyman-α in atomic hydrogen

Second-harmonic generation in atomic hydrogen is investigated with and without an applied electric field, by tuning laser radiation at 243.1 nm through the 2 (2)S((1/2))-1(2)S((1/2)) two-photon resonance. Even at zero applied field, secondharmonic radiation at 121.6 nm was observed. The measured intensity is shown to arise from a charge-separation field caused by ions produced in three-photon ionization. calculated value based on the measured ion density.

Resonance-enhanced multiple-photon ionisation spectroscopy of excited sulphur atoms

Sulfur atoms are produced in the excited 3s23p41D2 and 3s23p41S0 levels by two-photon photodissociation of CS2 in the gas phase in the region 285–305 nm. These excited atoms are detected by three-photon (two to resonance) ionisation at selected laser wavelengths. Many new transitions have been observed which have not been reported previously. The new “dark” states that have been accessed by two-photon absorption have been characterised and the energy levels with respect to the ground 3P2 state have been determined. Configuration interaction between 6p3P1, 6p5P1, and 4p′1P1 states, and also between 6p3P2, 6p5P2, and 4p′1D2 states has been observed. It is found that intermediate states reached from the 1S0 level through two-photon absorption lie above the first ionisation potential of S+(4S0). It is proposed that autoionisation into the 4S0+e− continuum is a dominant mechanism in the decay of these levels, although ionisation by a further photon absorption is not ruled out.

Resonant three-photon ionization and two-photon laser-induced fluorescence of atomic oxygen in a discharge and post-discharge

A method is presented which would allow the determination of the absolute number density of O atoms in discharge and post-discharge by simultaneous measurements of the number of photoions created from 2 + 1 resonant-enhanced multiphotoionization (REMPI) of atomic oxygen and the LIF (laser-induced fluorescence) from two-photon excitation. The REMPI technique is used in post-discharge, leading to the determination of the number density of O atoms in depletion condition in the interaction volume. The LIF technique is used in the same discharge conditions, allowing then the calibration which would be used to deduce the absolute number density of O atoms in a discharge.

Two-photon spectroscopy of Rydberg states of molecular oxygen

Two-photon resonant, three-photon ionization spectrum of O2X 3Σ−g, v″=0 is presented in the energy region between the 5s–4d complex and the O+2X 2Π1/2g, v+=0 ionization threshold. The spectrum exhibits a number of new transitions to ns and nd Rydberg states with n=5-9 and v′=0 and 1 as well as transitions to the previously unobserved 5s–4d, v′=3 levels. For a given value of n and v′, the observed bands split into two groups separated by approximately the O+2X 2Π1/2g−2Π3/2g spin–orbit splitting. All of the bands show at least some sharp rotational structure, although in most cases this structure is limited to a few lines. In addition, the direct two-photon ionization spectrum of O2X 3Σ−g, v″=0 is presented in the energy region between the O+2X 2Π1/2g, v+=0 and 1 thresholds; the spectrum shows transitions to Rydberg states with n=8-12 and v′=1. These states are analogous to the v′=0 states observed below the ionization threshold, and they decay by vibrational autoionization into the v+=0 continuum. Taken together, the v′=0 and 1 Rydberg states observed here provide a significant addition to the knowledge of Rydberg series converging to the ground electronic state of O+2.