Temporal phase control over the multi-photoionization of NO is investigated experimentally. The coherent excitation of the molecular system is achieved by a sequence of two phase-related non-Fourier-transform-limited laser pulses delivered by a nanosecond dye laser. The phase-locked pulses are produced by making use of an actively stabilized interferometer with a tunable path-length difference. As the coherence time of the laser is comparable with the inhomogenous dephasing time of the medium, the measurements are performed with temporally overlapped pulses. In order to avoid the modulation of the ionic signal coming from optical interference, the linear polarizations of the two pulses are crossed. The quantum modulation of the signal versus the time delay between the two pulses is observed for the three following distinct processes: (a) two-photon resonant four-photon ionization of the ground vibronic state; (b) two-photon absorption of the A-X transition followed by ionization; and (c) two-photon ionization of the electronically excited state A. As a result, temporal phase control over bound-bound and bound-free transitions is demonstrated. The efficiency of the control process is shown to be limited by the presence of uncoupled excitation channels inherent to the method employed. The advantage in using a sequence of cross-polarized pulses for the controlling processes is discussed.
Temporal phase control over the multi-photoionization of NO is investigated experimentally. The coherent excitation of the molecular system is achieved by a sequence of two phase-related non-Fourier-transform-limited laser pulses delivered by a nanosecond dye laser. The phase-locked pulses are produced by making use of an actively stabilized interferometer with a tunable path-length difference. As the coherence time of the laser is comparable with the inhomogenous dephasing time of the medium, the measurements are performed with temporally overlapped pulses. In order to avoid the modulation of the ionic signal coming from optical interference, the linear polarizations of the two pulses are crossed. The quantum modulation of the signal versus the time delay between the two pulses is observed for the three following distinct processes: (a) two-photon resonant four-photon ionization of the ground vibronic state; (b) two-photon absorption of the A-X transition followed by ionization; and (c) two-photon ionization of the electronically excited state A. As a result, temporal phase control over bound-bound and bound-free transitions is demonstrated. The efficiency of the control process is shown to be limited by the presence of uncoupled excitation channels inherent to the method employed. The advantage in using a sequence of cross-polarized pulses for the controlling processes is discussed.
In this study, we extended the coherent control theory proposed by Brumer and Shapiro to bound-continuum transition in a molecular system where control of the ejection direction of photoelectrons in the ionization of nitric oxide was achieved. Nitric oxide was prepared in its first excited Rydberg state and was then ionized simultaneously by two different paths: a one-photon and a two-photon process. The fundamental and second harmonic frequencies of a dye laser were used for the two different ionization processes. Interference due to the two paths was detected by measuring the photoelectrons ejected in a defined solid angle. Changing the relative phase between the two paths resulted in a destructive or constructive interference leading to a decrease or increase of the measured signal, respectively. This study showed that interference experiments are very sensitive, not only to lasers' properties and the optical components, but also to the properties of the molecular system. Laser properties, such as, spatial and temporal mode qualities, power fluctuations and timing, influenced the relative strength of the signals of the two ionization paths. Optical component properties, such as, surface parallelism and quality, affected laser divergence, and therefore, considerably reduced the depth of modulation. As for the molecular system, in a resonance two-photon excitation, appropriate resonance states must be chosen to ensure that the two processes produce electron waves of opposite parities which can interfere constructively to produce an enhancement of electron ejection in a certain direction. Due to the non-ideal properties of the lasers used, it is best to use a non-resonant two-photon ionization as one of the interfering paths. Resonant two-photon ionization, combined with the multi-mode laser greatly reduced the depth of the modulation.
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.
We theoretically study the control of two-photon excitation to bound and dissociative states in a molecule induced by trains of laser pulses, which are equivalent to certain sets of spectral phase modulated pulses. To this end, we solve the time-dependent Schr\"odinger equation for the interaction of molecular model systems with an external intense laser field. Our numerical results for the temporal evolution of the population in the excited states show that, in the case of an excited dissociative state, control schemes, previously validated for the atomic case, fail due to the coupling of electronic and nuclear motion. In contrast, for excitation to bound states the two-photon excitation probability is controlled via the time delay and the carrier-envelope phase difference between two consecutive pulses in the train.
In this paper, we theoretically investigate the control of the non-resonant two-photon absorption induced by two time-delayed laser pulses, and an analytical solution for the dependence of the two-photon transition probability on the time delay and relative carrier-envelope phase of the two laser pulses is achieved. We show that the two-photon absorption can be significantly enhanced or completely suppressed by varying the time delay between the two laser pulses or their relative carrier-envelope phase. We also show that the two-photon absorption can be selectively excited when two excited states are simultaneously excited. Furthermore, we discuss the two-photon absorption control in the molecular system and analyze the effect of the absorption bandwidth on the control efficiency of the two-photon absorption.
We experimentally demonstrate one- and two-photon phase sensitive coherent control over the excitation and ionization of Rydberg states of Sr atom in the presence of a static homogeneous electric field. Without the field this excitation scheme can only be employed for the manipulation of angular distributions of photoelectrons and molecular photofragmentation products. Total atomic excitation-ionization yields cannot be modulated because the final states excited by each pathway are orthogonal to each other. When, however, a static electric field is applied, s, p and d character is admixed into the final Rydberg state, which has no definite parity anymore. Hence, the excitation of the target Stark state from the ground state is possible with either two (fundamental laser frequency) photons or one (second harmonic frequency) photon and its population and further ionization can be controlled by varying the relative phase between the two radiation fields. The concept is successfully tested below as well as above the classical saddle point and with either mutually crossed or parallel linear laser beam polarizations (while the second harmonic beam polarization vector is always parallel to the static field direction). We examine the behavior of the obtained photoionization signal modulation depth V as a function of the static field strength F for otherwise identical experimental conditions. The V(F) curve exhibits a maximum (typically $\ensuremath{\sim}65%--85%$) at a field strength value that is dictated by the interplay between the employed laser power densities, one- and two-photon transition dipole moments, and relative amounts of field-dependent s, p and d character. It is therefore shown that the static field strength may serve as an additional, experimentally adjustable, control parameter in a fashion complementary to both the intensities and relative phase of the two light beams.
We present an experimental and theoretical investigation of the quantum control of ground-state rotational coherence in a linear molecule. A sequence of two temporally separated laser pulses creates a rotational superposition state in CO2 whose evolution is monitored through a polarization technique. We study the influence of the phase difference between the two pulses. We show that the overlapping of the two wave packets, produced by each pulse, gives rise to quantum interference that affects the orientational anisotropy of the sample. Because of the large number of coherently excited levels, the interference produces well-separated temporal structures, whose magnitude can be controlled through the relative delay of the two pulses.