Peter M. Weber

Ultrafast electron microscopy in materials science, biology, and chemistry

The use of pump-probe experiments to study complex transient events has been an area of significant interest in materials science, biology, and chemistry. While the emphasis has been on laser pump with laser probe and laser pump with x-ray probe experiments, there is a significant and growing interest in using electrons as probes. Early experiments used electrons for gas-phase diffraction of photostimulated chemical reactions. More recently, scientists are beginning to explore phenomena in the solid state such as phase transformations, twinning, solid-state chemical reactions, radiation damage, and shock propagation. This review focuses on the emerging area of ultrafast electron microscopy (UEM), which comprises ultrafast electron diffraction (UED) and dynamic transmission electron microscopy (DTEM). The topics that are treated include the following: (1) The physics of electrons as an ultrafast probe. This encompasses the propagation dynamics of the electrons (space-charge effect, Child’s law, Boersch effect) and extends to relativistic effects. (2) The anatomy of UED and DTEM instruments. This includes discussions of the photoactivated electron gun (also known as photogun or photoelectron gun) at conventional energies (60–200 keV) and extends to MeV beams generated by rf guns. Another critical aspect of the systems is the electron detector. Charge-coupled device cameras and microchannel-plate-based cameras are compared and contrasted. The effect of various physical phenomena on detective quantum efficiency is discussed. (3) Practical aspects of operation. This includes determination of time zero, measurement of pulse-length, and strategies for pulse compression. (4) Current and potential applications in materials science, biology, and chemistry. UEM has the potential to make a significant impact in future science and technology. Understanding of reaction pathways of complex transient phenomena in materials science, biology, and chemistry will provide fundamental knowledge for discovery-class science.

Ultrafast time-resolved electron diffraction with megavolt electron beams

A rf photocathode electron gun is used as an electron source for ultrafast time-resolved pump-probe electron diffraction. The authors observed single-shot diffraction patterns from a 160nm Al foil using the 5.4MeV electron beam from the Gun Test Facility at the Stanford Linear Accelerator. Excellent agreement with simulations suggests that single-shot diffraction experiments with a time resolution approaching 100fs are possible.

Intramolecular vibrational relaxation in the S0 state of s‐tetrazine–X (X=Ar, Kr, Xe)

A triple resonance experiment with fluorescence detection has been carried out to investigate the spectroscopy and intramolecular dynamics of the van der Waals molecules s‐tetrazine–X (X=Ar, Kr, Xe) with vibrational energies from 1250 to 2210 cm−1 in the ground electronic state (S0). Vibrational band shifts in S0 van der Waals molecules are usually on the order of 1 cm−1, and most relaxation lifetimes are longer than 15 ns. Comparison with published data for intramolecular vibrational redistribution in the S1 electronic state indicates a strong influence of the electronic structure of the aromatic ring on the dissociation dynamics, in clear contradiction to statistical models which have been advanced previously. A perturbation theory treatment supports the existence of a correlation between vibrational band shifts and dissociation dynamics, and rationalizes qualitatively all the peculiar observations for this class of van der Waals molecules.

Time‐delayed two‐color photoelectron spectra of aniline, 2‐aminopyridine, and 3‐aminopyridine: Snapshots of the nonadiabatic curve crossings

We present time‐delayed two‐color photoionization photoelectron spectra of aniline, 2‐aminopyridine, and 3‐aminopyridine seeded in a cold molecular beam. The molecules are prepared in their S1 electronic states by a picosecond UV laser pulse and ionized by a time‐delayed 200 nm probe pulse. The photoelectron spectrum is observed with a time‐of‐flight spectrometer. All time‐delayed spectra reveal only one product of the nonradiative relaxation process. Careful considerations of electronic and vibrational overlap propensity rules for the ionization step lead to the conclusion that the dominant nonradiative decay mechanism in these molecules is the intersystem crossing to a bath of vibrationally excited levels of the T1 electronic state. Our observations reveal no admixtures of T2 or higher triplet levels. The pathway of the nonradiative electronic relaxation in 2‐aminopyridine is found to be independent of the initially prepared vibrational states up to 1000 cm−1 of vibrational energy. We find no evidence o...

Continuous-wave oscillation of thulium-sensitized holmium-doped silica fiber laser.

We have observed oscillation in a Tm(3+) and Ho(3+) codoped silica fiber from the (5)I(7) ? (5)I(8) transition of Ho(3+) in the 2-microm region, by pumping the fiber laser into a Tm(3+) absorption between 800 and 830 nm. By a change in the cavity length, the laser was tunable between 2.037 and 2.096 microm. When the laser was pumped with a Ti:sapphire laser at 820 nm, an absorbed threshold power extracted power of 214mW, a slope efficiency of 4.2%,and maximum extracted power of 12.5 mW were measured.

The Ultrafast Pathway of Photon-Induced Electrocyclic Ring-Opening Reactions: The Case of 1,3-Cyclohexadiene

The photochemically induced electrocyclic ring-opening reaction of 1,3-cyclohexadiene to 1,3,5-hexatriene serves as a prototype for many important reactions in chemistry and in biological systems. Based on experimental and computational studies, a detailed picture of the reaction has now emerged: Excitation to the Franck-Condon region places the molecule on a steeply repulsive part of the 1B potential energy surface, which propels the molecule in exactly the conrotatory direction that conforms to the Woodward-Hoffmann rules of orbital symmetry. Bypassing a cusp in a symmetry-breaking direction, the wave packet enters the 2A state within 55 fs. It continues to move directly toward the 2A/1A conical intersection, where it crosses in approximately 80 fs to the ground state. This article summarizes the published experimental and theoretical work to describe the current understanding of the reaction while pointing to important questions that remain to be addressed.

Spectroscopy and femtosecond dynamics of the ring opening reaction of 1,3-cyclohexadiene.

The early stages of the ring opening reaction of 1,3-cyclohexadiene to form its isomer 1,3,5-hexatriene, upon excitation to the ultrashort-lived 1 1B2 state, were explored. A series of one-color two-photon ionization/photoelectron spectra reveal a prominent vibrational progression with a frequency of 1350 cm(-1), which is interpreted in a dynamical picture as resulting from the ultrafast wave packet dynamics associated with the ring opening reaction. Photoionization in two-color three-photon and one-color four-photon ionization schemes show an ionization pathway via the same ultrashort-lived 1 1B2 state, and in addition, a series of Rydberg states with quantum defects of 0.93, 0.76, and 0.15, respectively. Using those Rydberg states as probes for the reaction dynamics in a time-resolved pump-probe experiment provides a direct observation of the elusive 2 1A1 state that has been implicated as an intermediate step between the initially excited 1 1B2 state and the ground electronic state. The rise and decay times for the 2 1A1 state were found to be 55 and 84 fs, respectively.

Effect of Pulse Shape on the Efficiency of Multiphoton Processes: Implications for Biological Microscopy

The effects of spectral shape on two photon fluorescence excitation are investigated experimentally using an acousto-optic pulse shaper to modify femtosecond pulses from a Ti:sapphire laser. By using different spectral window shapes, we find that the measured two photon efficiency can vary by a factor of 2 for differently shaped spectra with the same full width at half maximum. We find that these effects are described well by a simple model assuming transform-limited pulses. The fact that even small changes in the spectral wings can significantly affect the efficiency of nonlinear processes has implications for biological multiphoton imaging, where it may be desirable to minimize sample exposure to radiation and maximize fluorescence or harmonic efficiency.

High resolution photoelectron spectroscopy: The vibrational spectrum of the 2‐aminopyridine cation

We describe a new time‐of‐flight photoelectron spectrometer that combines molecular beam techniques with two‐photon ionization by a high repetition rate laser. The instrument routinely achieves a resolution of 5 meV. In a first application we studied the vibrational spectrum of the 2‐aminopyridine ion. By tuning the laser to various vibrational states of the intermediate S1 resonance we were able to assign the vibrational frequencies of modes 6a, 12, I2 and 1. Other vibrational lines could be assigned by comparison with similar molecules. The ionization potential was found to be 8.099±0.003 eV, which differs from the literature value by 0.124 eV. The discrepancy might be explained by a fast intersystem crossing to a nearby triplet state.

van der Waals bond stretch and bend frequencies in the molecules tetrazine–X (X=Xe, Kr, and Ar)

Fluorescence excitation and dispersed fluorescence spectra of the clusters s‐tetrazine–X (X=Xe, Kr, Ar) have been studied to determine the van der Waals bond stretch and bend frequencies. In the case of tetrazine–Xe a vibrational progression spanning v=0 to 8 in the bend vibrations was detected. The harmonic frequencies of the van der Waals bond stretch and bond bend vibrations are found to be very similar for the T–Xe, T–Kr, and T–Ar clusters. Spectra of the symmetric trimer xenon–tetrazine–xenon show progressions in the van der Waals bond stretch vibration up to v‘=5. The frequencies of the stretch vibration in tetrazine–xenon and the symetric stretch vibration in xenon–tetrazine–xenon support the validity of pairwise additivity of the van der Waals potentials. The excitation spectra also indicate the prescence of other, more complicated, clusters of tetrazine with xenon.