Sergei A. Trushin

Time-resolved dissociative intense-laser field ionization for probing dynamics: Femtosecond photochemical ring opening of 1,3-cyclohexadiene

The concerted photochemical ring opening of 1,3-cyclohexadiene was investigated in the gas phase by low-intensity pumping at 267 nm and subsequent probing by high-intensity photoionization at 800 nm and mass-selective detection of the ion yields. We found five different time constants which can be assigned to traveling times along consecutive parts of the potential energy surfaces. The molecule is first accelerated in the spectroscopic state 1B along Franck–Condon active coordinates, then alters direction before changing over to the dark state 2A. All constants including that for leaving the 2A surface are below 100 fs. These times are shorter than appropriate vibrational periods. Such a maximum speed is evidence that the pathway is continuous leading from surface to surface via real crossings (conical intersections) and that the molecule is accelerated right into the outlet of the 2A/1A funnel. On the ground state it arrives as a compact wave packet, indicating a certain degree of coherence. The experime...

Pathway approach to ultrafast photochemistry: potential surfaces, conical intersections and isomerizations of small polyenes

Abstract The dynamics of molecules, attention to which is being stimulated by modern ultrafast spectroscopy, as well as progress in quantum chemistry are giving new impetus to the approach which follows the detailed pathway of the molecules along the potential energy surfaces from the excited to the ground state. Local properties such as slopes, saddle points, barriers and funnels with conical intersections as exits are taken into account. We show the fruitfulness of this approach by means of new and published experimental results on photochemical pericyclic reactions. The experimental data support the importance of conical intersections and, in some cases, yield a very detailed picture of the dynamics on the potential energy surfaces. We emphasize the relation of the pericyclic potential surfaces to those of cis–trans isomerization and show that, in this case, the geometries of the conical intersections are interrelated and can be intuitively understood. The conical intersections of stilbene and polyenes can be deduced from the prototype of ethylene, and their geometries are supported by photochemical evidence.

FEMTOSECOND PHOTOCHEMICAL RING OPENING OF 1,3-CYCLOHEXADIENE STUDIED BY TIME-RESOLVED INTENSE-FIELD IONIZATION

We found that hydrogen ion formation due to multielectron dissociative ionization by an intense-laser field is much less efficient with 1,3-cyclohexadiene than with its isomer 1,3Z,5-hexatriene (Z-HT). Moreover by suppressing the ionization barrier an intense-laser field ejects electrons most efficiently from molecular states of low ionization potential. After pumping 1,3-cyclohexadiene at 267 nm to its 1B2 state we probe the system by intense-laser field ionization with delayed 800 nm pulses. Monitoring of the parent ion C6H8+, of the main fragment C6H7+ and of H+ allows us to follow the motion from the 1B2 surface to the dark 2A1 state and from there towards the 2A1/1A1 conical intersection to the ground-state surface of the product. The measured 1B2 and 2A1 lifetimes are 43±3 and 77±7 fs, respectively, and the primary photoproduct cZc-HT is produced within 200 fs.

Conical intersections, pseudorotation and coherent oscillations in ultrafast photodissociation of group-6 metal hexacarbonyls

Abstract By time-resolved nonresonant (800 nm) multiphoton ionization we found five consecutive processes and pronounced coherent oscillations after excitation of M(CO) 6 , M=Cr, Mo and W at 267 nm in the gas phase. We suggest that the first two steps correspond to relaxation along a Jahn–Teller (JT) active coordinate and internal conversion between metal-to-ligand charge-transfer states via a JT induced conical intersection, whereas in the third step the molecules change over to a repulsive ligand-field surface and dissociate. The primary product is M(CO) 5 in its S 1 state which can again relax in an ultrashort time through a JT induced conical intersection to S 0 ; the JT active coordinate (relaxation pathway) corresponds to pseudorotation. The total time to reach S 0 takes 110, 165 and 195 fs for the three carbonyls (calculated from the measured first four time constants). After arrival at S 0 , M(CO) 5 oscillates coherently along a pseudorotation coordinate. In S 0 , M(CO) 5 eliminates a second CO in about 1 ps owing to its vibrational excess energy, a step which is suppressed in solution by cooling. All processes take place in the singlet manifold of states.

Vacuum ultraviolet pulses of 11 fs from fifth-harmonic generation of a Ti:sapphire laser

We demonstrate that in a short Ar cell, generation of the fifth harmonic from 12 fs pulses at 810 nm directly results in ultrashort vacuum UV pulses at 162 nm. They have a spectral width of approximately 5 nm and a duration of 11+/-1 fs (1.4 times the transform limit), as measured by cross correlation with the fundamental pulses. Their energy (estimated to 4 nJ) turned out to be sufficient for use as a pump in time-resolved experiments.

Sub-10-fs supercontinuum radiation generated by filamentation of few-cycle 800 nm pulses in argon

Focusing 12 fs pulses of 800 nm with moderate energy (0.35 mJ) into atmospheric-pressure argon (Ar) gives rise to filamentation (self-focusing) and a supercontinuum with a very broad pedestal, extending to 250 nm. According to the present understanding, the short wavelengths are produced by self-phase modulation in the self-steepened trailing edge of the pulse. Pulses in this spectral range might thus be intrinsically short. Indeed we demonstrate this by extracting the light near the end of the filament, terminating self-focusing by a pressure gradient at a pinhole, beyond which the Ar is pumped away. We obtain pulses of 9.7 fs in the region of 290 nm without the necessity of compression.

Dissociative ionization at high laser intensities: importance of resonances and relaxation for fragmentation

We investigated dissociative single and double ionization of the metal carbonyls Ni(CO)4, Fe(CO)5 and Cr(CO)6 in the gas phase by means of laser pulses of different durations (30–110 fs) and wavelengths (0.8 and 1.35 µm) at intensities of 2 × 1012–2 × 1014 W cm−2. The mass spectra show striking differences: for example, Fe(CO)5 strongly fragments at 0.8 µm but little at 1.35 µm, whereas for Ni(CO)4 fragmentation is higher at 1.35 µm than at 0.8 µm; chromium carbonyl shows little fragmentation at both wavelengths. In other cases, fragmentation first decreases and then increases again with intensity. These and other phenomena, also published ones, can readily be understood from long-known principles, namely resonances in the parent ions, sometimes also in the neutral molecules, in particular if relaxations are also taken into account. We emphasize that fragmentation and ionization are two separate processes. We also point out that in the process of dissociative ionization in intense laser radiation, one should generally consider intermediate states, even if there is no one-photon resonance.

High energy picosecond Yb:YAG CPA system at 10 Hz repetition rate for pumping optical parametric amplifiers

We present a chirped pulse amplification (CPA) system based on diode-pumped Yb:YAG. The stretched ns-pulses are amplified and have been compressed to less than 900 fs with an energy of 200 mJ and a repetition rate of 10 Hz. This system is optically synchronized with a broadband seed laser and therefore ideally suited for pumping optical parametric chirped pulse amplification (OPCPA) stages on a ps-timescale.

Extending the supercontinuum spectrum down to 200 nm with few-cycle pulses

By focusing 805 nm pulses of low energy (0.2-1 mJ) into atmospheric-pressure argon, a supercontinuum is generated with a short- wavelength cutoff of 640, 250 and 210 nm for initial pulse durations of 45, 10 and 6 fs, respectively. It is shown numerically that the large shift of the UV cutoff and many features of the spectrum are caused by terms beyond the slowly-varying- envelope approximation (SVEA). Their effect on pulse compression and filament length is also discussed.

Ultrafast dynamics of the photochemical ring opening of 1,3-cyclohexadiene studied by multiphoton ionization

Femtosecond time-resolved studies of the ring opening of 1,3-cyclohexadiene are presented. After absorption of a single UV photon in the region 250–270 nm (1A1 → 1B2 transition), the reaction is investigated using time-delayed single- and multiphoton ionization with probe pulses in the region 250–415 nm. Ions are detected by a time-of-flight mass spectrometer. The parent ion is only observed during the time when the pump and probe lasers overlap. The corresponding ionizable state, which we identify as 1B2, has a short lifetime which we estimate to be 60 fs. The only signal detected after a delay is due to C6H7+. It arises from dissociation of vibrationally hot parent ions which are produced by two-photon ionization of vibrationally excited products. The appearance rate constant of the product is as high as 1.7 ± 0.2 ps−1. It is assigned to the transition from the 2A1 to the 1A1 surface via a conical intersection, i.e. to the photochemical ring opening of 1,3-cyclohexadiene to Z-hexatriene. Measuring fragment ions generated by photoionization at suitable wavelengths may be a general method for monitoring vibrationally hot neutral molecules.