Langchi Zhu

Coherent Laser Control of the Product Distribution Obtained in the Photoexcitation of HI

Active control of the distribution of products of a chemical reaction was demonstrated by using a method based on the principle of quantum mechanical interference. Hydrogen iodide (HI) molecules were simultaneously excited above their ionization threshold by two competing pathways. These paths were absorption of three ultraviolet photons of frequency ω1 and one vacuum ultraviolet photon of frequency ω3 = 3ω1. The HI+ and I+ signals were modulated as the phase between the lasers was varied, with the HI+ signal lagging by 150° ± 15°. A mechanism consisting of autoionization and predissociation is proposed.

Coherent phase control of the photoionization of H2S

Coherent phase control was demonstrated for a bound‐to‐continuum transition of a polyatomic molecule. Three UV photons of frequency ω1 and one vacuum ultraviolet (VUV) photon of frequency ω3=3ω1 simultaneously excited H2S above its ionization threshold. The parent ion, H2S+, and fragment ions, HS+ and S+, produced by absorption of additional photons, were observed. All three ion signals were modulated as the phase difference between the light fields was varied.

Coherent control over the photodissociation of CH3I

Coherent phase control of the photodissociation of CH3I has been achieved by quantum mechanical interference between competing paths. The control was accomplished by exciting the parent molecules with three UV photons of frequency ω1 and one VUV photon of frequency ω3=3ω1. Varying the phase difference between the two laser beams resulted in a modulation of the I+ and CH+3 signals, without affecting the parent ion signal. We propose a mechanism in which control occurs over the photodissociation step to produce CH3+I*, followed by ionization of the neutral fragments by additional UV photons.

Nanolithography using molecular optics

We explore the possibility of using an intense laser beam to focus a molecular beam onto a surface to create nanowires. We show that with a grazing angle of incidence between the laser and molecular beams, it is possible to use available technology to create wires 100 μm long with a 100 W continuous wave laser. Narrower and longer features could be created with higher power lasers. This technique is very general, and may be used to deposit any atom or molecule onto an arbitrary substrate, so long as the particles may be entrained in a molecular beam and have an adequate sticking probability. The effects of spherical and chromatic aberration and laser mode structure on the focusing properties of the molecular lens are examined in detail, and design criteria for building a practical device are discussed.

The use of coherent phase control of multiphoton ionization to measure the refractive indices of H2 and Ar between 1100 and 1150 Å

We have used the method of coherent phase modulation, first proposed by Brumer and Shapiro for controlling the rates of chemical reactions, to measure the refractive index of a gas in the vacuum ultraviolet. In these experiments we populated a Rydberg state of HCl or CO by simultaneous absorption of one VUV photon and three UV photons. Molecular ions were generated by absorption of an additional UV photon and detected with a time‐of‐flight mass spectrometer. The phase difference for the UV and VUV beams was altered by passing them through a cell containing either H2 or Ar. By varying the pressure in the cell we could control this phase difference and thereby modulate the ion signal. From the modulation frequency of the signal it was possible to determine the difference between the refractive indices of the gas at the UV and VUV wavelengths. Using reliable refractive indices in the UV, we extracted from our data values for the VUV indices. These values are in quantitative agreement with a calculation that ...

Mechanism of the coherent control of the photoionization and photodissociation of HI and DI

Abstract Two experiments were performed to determine the mechanism responsible for the phase lag between the HI + and I + signals observed by Zhu et al. (Science, 270 (1995) 77) in the one- versus three-photon coherent control of the decay of excited HI and DI. In the first experiment a pulse of 266 nm radiation was introduced before the UV and VUV control pulses. It was observed in this case that the modulation amplitudes of the I + signal decreased slightly, whereas the modulation depths decreased by a factor of two. This experiment rules out the possibility that modulation of I + is caused by coherent control of the ionization of I atoms generated by two-photon photodissociation of the parent molecule. In the second experiment a fluorescence excitation spectrum produced by a single VUV photon was recorded and is attributed to emission from the atomic iodine product. Both experiments support our original interpretation of the phase lag arising from coherent control of the branching ratio for autoionization versus predissociation of the excited parent molecule.

The role of molecular and resonance phases in the coherent control of chemical reactions

Coherent control of the photoionization and photodissociation of HI and the photoionization of H2S were obtained in the region of the 5d(π, δ) resonance of HI. Interference between one- and three-photon excitation paths caused modulation of the HI+, I+, and H2S+ signals. Phase lags between the different modulated signals, measured as a function of excitation energy, revealed the roles played by molecular and resonance phases. A theory of the phase lag arising from a set of overlapping rotational resonances was developed and used to interpret the observations.

Study of isotope effects in the photoionization of HI and DI using phase lag spectroscopy

Phase lag spectroscopic measurements are performed for HI and DI in the 5d(π,δ) resonance region. Measurements of the phase lags for both molecules reveals a strong isotopic effect, especially pronounced in the ionization channel. The energy dependence of the phase lag provides clear examples of continuum and resonance coupling effects that were previously predicted theoretically but have not yet been observed. It is demonstrated that phase lag spectroscopy is a sensitive technique for studying properties of molecular continua, furnishing information that is not available from phase-insensitive observables.

Coherent Phase Control of Photoionization and Photodissociation

A fundamental principle of quantum mechanics is that if a process can occur by more than one independent path, then the probability of that process occurring can be calculated by adding the probability amplitudes for each path and then squaring the sum. A well known example is the photoionization of an atom or a molecule. One route connecting the ground state g > with the continuum 39-1 > is direct ionization, with a probability amplitude that is proportional to 39-1