Cyclohexadiene ring opening observed with 13 fs resolution: coherent oscillations confirm the reaction path.

The third harmonic (270 nm, 11 fs), produced in a short argon cell from Ti-sapphire laser pulses (810 nm, 12 fs), was used to excite 1,3-cyclohexadiene to its lowest pipi* state (1B). Probing was done by transient ionization by the 810 nm pulses, measuring the yields of the parent and a fragment ion. As previously found with 10 times longer pulses, the molecule leaves in two steps (time constants tau(1), tau(2)) from the spectroscopic (1B) to a dark (2A) state and from there (within tau(3)) to the ground-state surface. In addition to slightly improved values for tau(1)-tau(3), we found in all three locations (L(1)-L(3)) on the potentials coherent oscillations, which can be assigned to vibrations. They are stimulated by slopes (driving forces) of the potentials, and the vibrational coordinates indicate the slope directions. From them we can infer the path following the initial excitation: the molecule is first not only accelerated towards CC stretching in the pi system but also along a symmetric C[double bond, length as m-dash]C twist. The latter motion-after some excursion-also erects and stretches the CH(2)-CH(2) bond, so that Woodward-Hoffmann interactions are activated after this delay (in L(2)). On leaving L(2) (the 1B minimum) around the lower cone of the 1B/2A conical intersection, the wave packet is rapidly accelerated along an antisymmetric coordinate, which breaks the C(2) symmetry of the molecule and eventually leads in a ballistic path to (and through) the last (2A/1A) conical intersection. The ring opening begins already on the 1B surface; near the 2A minimum it is already far advanced, but is only completed on the ground-state surface.

Ultrafast dynamics and coherent oscillations in ethylene and ethylene-d4 excited at 162 nm.

The fifth harmonic (162 nm, 11 fs), generated in a short argon cell from 12 fs Ti-sapphire laser pulses, was used to excite C2H4 and C2D4 in the maximum of the first pi pi* transition. Around 10% of the molecules were excited to the pi3s Rydberg state instead. The subsequent motion of the wave packet, moving over the potentials from the Franck-Condon region down to the ground state, was monitored by nonresonant ionization at 810 nm with mass-selective detection of the ion yield. Five time constants (from approximately 20 fs in excited states to 0.6-11 ps in the hot ground state) and four coherent oscillations (CC stretch and torsion vibrations or hindered free rotation) were determined for each isotopomer. The initial relaxation follows a superposition of CC twist and stretch coordinates; this explains a surprisingly small deuterium isotope effect of the initial time constant (21 versus 24 fs). Also the vibrations in the Franck-Condon region have such a mixed character and a correspondingly small isotope shift. From the perpendicular minimum the wave packet reaches (within 17 or 21 fs for the two isotopomers) a conical intersection via a direction that also involves partial hydrogen migration. This is concluded from the detection of ethylidene (CH3CH), formed simultaneously with ground-state ethylene. This carbene isomerizes in the ground state within 0.6 ps (1.6 ps for CD3CD) to ethylene. Two time constants for dissociation (4.5 and 11 ps) in the hot ground state were also identified. The small yields of bimolecular reactions (photodimerization, addition reactions involving a "suddenly polarized" excited state, carbene reactions) are interpreted in terms of the short lifetimes. It is pointed out that the relaxation path starting from the Rydberg state merges into that from the pi pi* state; nevertheless, there is a wavelength dependence in the photochemistry of olefins, because due to a momentum effect the wave packet remembers from which state it came.

Ultrafast dynamics of cyclohexene and cyclohexene-d10 excited at 200 nm.

By exciting cyclohexene in the gas phase at 200 nm and probing it by nonresonant multiphoton ionization with mass-selective detection of the ion yields, we found four time constants tau(i) (20, 47, 43, 350 fs). Whereas deuteration lengthens tau2 by a factor of 1.4, the other constants do not change. Tau1-tau3 represent traveling times through observation windows on excited surfaces, whereas tau4 reflects a process in the hot ground state. We assign tau1 (20 fs) to departure from the Franck-Condon regions of the Rydberg and pipi* states, which are both populated at 200 nm, and tau2 (47 fs) to traveling along the pipi* surface and suggest that a [1,3]-sigmatropic H shift begins in this state. This rationalizes the deuterium effect on tau2. To explain why this window is followed by a process not subject to a D effect, we postulate that the pipi surface is crossed late (i.e., at low energy) by the zwitterionic state Z and that formation of a carbene (the known photochemical product, cyclopentylcarbene) begins there. The corresponding 1,2-shift of a CC bond is then (within tau4 = 350 fs) largely reversed on the ground-state surface, while a smaller part of the carbene forms products such as methylenecyclopentane within the same time. Carbene formation is probably accompanied by some cis-trans isomerization. The wavelength dependence of carbene formation is attributed to a memory for the initially excited state, based on momentum conservation. The processes are most likely typical of simple olefins. The fragmentation pattern showed that butadiene is not formed until at least 500 ps. The retro-Diels-Alder reaction, known to take place in the ground state, thus only occurs later.

Ion metabolism in a halophilic blue-green alga, Aphanothece halophytica.

The intracellular ion content of the halophilic blue-green alga, Aphanthece halophytica was studied as a function of age, external sodium and external potassium concentration. Intracellular Na+ was found to be about 0.38 millimoles/g dry mass. Intracellular K+ concentrations were as high as 1 M and varied directly with external salinity. Intracellular Ca++ and Mg++ were in the range previously reported for fresh water blue-green algae despite their extremely high extracellular concentrations. Average cell size is consistent at room temperature with two exceptions. When the outside K+ is lower than 6.5 mM the cells tend to be smaller with less intracellular K+ and high Ca++. In stationary phase cultrues the cells are larger with high intracellular Mg++ and low K+.