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The Journal of Chemical Physics

Modulation of resonant multiphoton ionization of CH3I by laser phase variation

Guoqiang Xing, Xue‐Bin Wang, Xin Huang, Richard Bersohn, B. Katz

Published Jan 15, 1996• 43 citations
Publisher

Abstract

The phase control of molecular absorption, first proposed by Shapiro, Hepburn, and Brumer was accomplished with CH3I using the technique of Chen, Yin, and Elliott. Red light (λ near 603 nm) was focused in a cell containing CH3I gas at a pressure around 1 Torr. The emerging light, a coherent mixture of the fundamental and the third harmonic (λ near 201 nm), was refocused on a molecular beam of CH3I and multiphoton ionization was detected. Ionization was by two simultaneous processes: a uv photon absorption followed by the absorption of two red photons (1+2 process) and an absorption of three red photons followed by the absorption of two more (3+2 process). Because of the sharp resonant Rydberg transition at 201 nm, the one and three photon matrix elements dominate the transition. Interference between the amplitudes of these two paths was demonstrated by varying the pressure and, hence, the index of refraction of the Ar gas in a tuning cell where the two light beams were refocused by two spherical mirrors. The modulation depth varied with wavelength but had a maximum of 75%, the largest modulation so far observed in this kind of experiment.

DOI
10.1063/1.470808

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The phase control of molecular absorption, first proposed by Shapiro, Hepburn, and Brumer was accomplished with CH3I using the technique of Chen, Yin, and Elliott. Red light (λ near 603 nm) was focused in a cell containing CH3I gas at a pressure around 1 Torr. The emerging light, a coherent mixture of the fundamental and the third harmonic (λ near 201 nm), was refocused on a molecular beam of CH3I and multiphoton ionization was detected. Ionization was by two simultaneous processes: a uv photon absorption followed by the absorption of two red photons (1+2 process) and an absorption of three red photons followed by the absorption of two more (3+2 process). Because of the sharp resonant Rydberg transition at 201 nm, the one and three photon matrix elements dominate the transition. Interference between the amplitudes of these two paths was demonstrated by varying the pressure and, hence, the index of refraction of the Ar gas in a tuning cell where the two light beams were refocused by two spherical mirrors. The modulation depth varied with wavelength but had a maximum of 75%, the largest modulation so far observed in this kind of experiment.
We have demonstrated an interference between multiphoton ionization processes that results in a phase-sensitive enhancement or inhibition of the net ionization rate. We focus two laser beams, either of which can induce an atomic transition between the same two states, into a cell containing atomic mercury. One of the fields is at a wavelength of 185 nm and can linearly induce the 6s 1S0 → 6p 1P1 transition in mercury. The other field is at a wavelength precisely three times as long, and it can excite the 6p resonance by a three-photon process. The absorption of two additional visible photons ionizes the mercury. In the presence of both fields, the transition amplitudes interfere, and their relative phases become very important. Their phases can be controlled by controlling the phases of the optical fields inducing the transitions. Coherence between the two fields, vital for the interference, is ensured by generating one field from the other using a nonlinear third-harmonic process. We have also demonstrated the effect of spatial phase variations between the two fields on the local photo-ionization rate.
Coherent control provides a quantum interference based method for controlling atomic and molecular dynamics. We report two experimental observations of different ways to achieve such a control in atomic photoionization continuum. One is the photoionization of sodium atoms, initially prepared in the 3P1/2 excited state, by a non resonant three-photon interaction with 1064-nm laser light and by a one-photon interaction with the third-harmonic field, at 355 nm. Interference between the two transition paths allows the control of the total ionization rate by varying the relative phase of the laser fields. The second experiment is the observation of the modification of the ionization continua of xenon through laser induced continuum structure (LICS). The multiphoton radiative decay of the xenon ground state into the two electronic continua, that correspond to the two fine structure levels of the ground state of the xenon ion, is varied by electromagnetically embedding a bound state of the atom into the two continua. Both experiments are performed on an atomic beam apparatus which uses a time-of-flight spectrometer for energy analyzing the emitted photoelectrons.
Multiphoton ionization in the region near a three-photon resonance is treated for focused, plane-polarized Gaussian beams with diffraction-limited beam divergence. In this situation, a third-harmonic field is generated within the laser beam. At, and very near, three-photon resonance the driving rate for the upper-state probability amplitude due to one-photon absorption of third-harmonic light becomes nearly equal to the corresponding three-photon rate due to the laser field, but these effects are 180\ifmmode^\circ\else\textdegree\fi{} out of phase. As a consequence of this cancellation between two pumping terms, the three-photon resonance line essentially disappears at moderate concentrations and the observed ionization has a line shape that is close to the phase-matching curve for third-harmonic generation. The ionization signal, near but not on the resonance, is due almost entirely to absorption of third-harmonic photons plus other laser photons; three-photon resonantly enhanced multiphoton ionization by the laser is much weaker. This is particularly true on the blue side of the three-photon resonance at detunings where phase matching occurs. The problem is treated quite generally with predictions of the full line shape for nnn-photon ionization and third-harmonic light generation near three-photon resonance, including the rather strong influences of positively dispersive buffer gases. We also show that the cancellation between the one-photon and the three-photon process is partially spoiled in the presence of a counterpropagating beam at the same frequency.
We report the observation of coherent control of the ionization rate in four-photon resonant, five-photon ionization schemes in Kr and Xe. The intermediate resonance is excited via different interfering channels involving four-photon absorption of the fundamental laser frequency ( ionization) as well as one third harmonic and one fundamental frequency photon absorption ( excitation), a particular aspect of the scheme being that interference occurs at a virtual level. By varying the relative phase of the two electromagnetic fields, a large modulation depth has been observed which is due to the chosen non-linear excitation scheme. In the Xe excitation experiment the interference between the different channels could be additionally probed through the modulation of the third-harmonic signal.
We consider effects which occur when four-wave sum frequency generation and multiphoton ionization are induced by lasers tuned near a three-photon resonance and simultaneously near or at a dipole allowed four-photon resonance. In studies with unfocused laser beams, if the phase mismatch of the generated four-wave-mixing field is large and the related two-photon resonance for the absorption of a four-wave-mixing photon and a laser photon results in strong absorption of the four-wave-mixing field, a coherent cancellation occurs between the pumping of the resonance by two- and four-photon processes. This interference effect occurs when the first laser is tuned on either side of the three-photon resonance and \ensuremath{\Vert}\ensuremath{\Delta}kr{\mathit{k}}_{\mathit{r}}kr​L\ensuremath{\Vert}\ensuremath{\gg}1, where \ensuremath{\Delta}kr{\mathit{k}}_{\mathit{r}}kr​ is the mismatch and L is the length of the path of the laser beams in the gas. With focused laser beams large differences occur between ionization with unidirectional beams and with counterpropagating laser beams when \ensuremath{\Vert}\ensuremath{\Delta}kr{\mathit{k}}_{\mathit{r}}kr​b\ensuremath{\Vert}\ensuremath{\gg}1, where b is the confocal parameter of the focused laser beams. Strong absorption of the four-wave-mixing field is shown not to be necessary for strong destructive interference with focused laser beams when the phase mismatch is large. This work also suggests an explanation for earlier experiments where the presence of a four-photon resonance enabled the generation of third-harmonic light in a positively dispersive wavelength region. We argue that this process can occur when the laser used to achieve the four-photon resonance is focused on the small z (z is the coordinate in the direction of propagation) side of the focal point of the laser responsible for the third-harmonic generation. In this way the absorption of the third-harmonic light on one side of the focal point is made much stronger than on the other side. Correspondingly, contributions to the third-harmonic field from the two sides of the focal point are unequal and cannot undergo complete destructive interference as would otherwise occur in a positively dispersive region.
The resonantly enhanced multiphoton ionization of atomic hydrogen is studied for the case of four-photon ionization (three-photon resonant). The time-dependent probabilities, the shape of the resonance profile at different intensities, and the nonlinear index are computed for coherent light at frequencies around the resonance (365 nm). The systematic dependence of the cross section as a function of the principal quantum number of the resonant state (as this tends towards the Rydberg states) is investigated at zero detuning. The probabilities of absorption of one photon more than necessary for the four-photon ionization process (resonantly enhanced above-threshold ionization) is computed and the relevant branching ratios are examined. Then it is shown that the effect of the spatio-temporal structures of the laser pulse is to red shift the maximum of the resonance profile, whereas that of an ideal chaotic light is to blue shift and broaden the resonance peak. The shape, shift, and width of the profile resulting from both these effects are in very fair agreement with the experimental ones.