In the past decade, the understanding of the dynamics of small molecules in intense laser fields has advanced enormously. At the same time, the technology of ultra-short pulsed lasers has equally progressed to such an extent that femtosecond lasers are now widely available. This review is written from an experimentalist's point of view and begins by discussing the value of this research and defining the meaning of the word 'intense'. It continues with describing the Ti : sapphire laser, including topics such as pulse compression, chirped pulse amplification, optical parametric amplification, laser-pulse diagnostics and the absolute phase. Further aspects include focusing, the focal volume effect and space charge. The discussion of physics begins with the Keldysh parameter and the three regimes of ionization, i.e. multi-photon, tunnelling and over-the-barrier. Direct-double ionization (non-sequential ionization), highharmonic generation, above-threshold ionization and attosecond pulses are briefly mentioned. Subsequently, a theoretical calculation, which solves the time-dependent Schrödinger equation, is compared with an experimental result. The dynamics of H + 2 in an intense laser field is interpreted in terms of bond-softening, vibrational trapping (bond-hardening), below-threshold dissociation and laser-induced alignment of the molecular axis. The final section discusses the modified Franck-Condon principle, enhanced ionization at critical distances and Coulomb explosion of diatomic and triatomic molecules.

Bond hardening of H 1 2 has been observed in the intensity range of 100 200 TW͞cm 2 using 792 nm laser pulses. This effect can be understood in terms of a light-induced potential well created at twice the normal (free) equilibrium internuclear distance by an adiabatic mixing of 1-and 3-photon resonances. The trapped population dissociates into H 1 and H when the potential well becomes convex on the trailing edge of the pulse. The dynamics of the nuclear wave packet was manipulated by chirping the pulse duration from 45 to 500 fs and observing a reduction of the kinetic energy release from 0.3 to 0.0 eV. This energy shift is interpreted as a dynamic Raman effect within the laser bandwidth. resonances were inferred from rather noisy data in the early 1990's. They were received with great interest, as at that time the stabilization of the molecular bond was a candidate for a universal mechanism explaining the invariance of ion kinetic energies with changes of intensity and pulse duration [5,6]. Later, it was established that this invariance is a signature of rapid, sequential ionization at the critical internuclear distance [7][8][9]. Since then there has been a surprising lack of clearcut confirmation of the bond hardening effect. With more recent work casting doubt on the existence of light-induced bound states [10,11], the idea of bond hardening has become again only a remote theoretical possibility. Against this trend of scepticism, we present an experimental observation of bond hardening in H 1 2 . In these experiments, chirped pulses from a Ti:sapphire laser were amplified to 10 mJ in energy and compressed to about 50 fs duration at a repetition rate of 10 Hz (see [12] for a recent review of high power ultrafast lasers). The pulse bandwidth had an almost perfect Gaussian shape centered at 792 nm and an extent of 22 nm, full width at half maximum (FWHM). The pulse length was varied by scanning the separation of the two compressor gratings and introducing some uncompensated chirp. The linearly polarized beam, 5 mm in diameter, was focused in an ultrahigh vacuum chamber using an f͞4 parabolic mirror to give a peak intensity of the order of 10 14 W͞cm 2 . Hydrogen gas was introduced into the vacuum chamber via simple effusion, raising the ambient pressure to about 10 26 torr. At this pressure and intensity no space charge effects were observed. Following the process of multiphoton ionization of H 2 , an external electric field directed "forward" and "backward" fragment ions into a vertical, 13-cm-long drift tube. Ions were detected by microchannel plates with a 10-mm circular restriction in front to improve the energy and angular resolutions. The ion time-of-flight (TOF) spectrum and pulse energy were recorded at each laser shot by a digital oscilloscope and stored in a computer. Figure 1 shows ion TOF spectra recorded at several pulse lengths. For each grating separation the peak intensity was adjusted to about 150 TW͞cm 2 , i.e., below saturation of any ion channel [13]. At each laser pulse the ion signal and t...

The angular distribution of the ionic fragments from multiphoton dissociative ionization of diatomic molecules shows a pronounced anisotropy. Calculations within the fieldionization, Coulomb explosion model show that the experimental I 2 results (pulses of 50 fs at an intensity of ∼10 14 W cm −2 ) can be explained without invoking reorientation of the molecular axis. The fragments of the lighter H 2 and N 2 molecules, on the other hand, show clear signs of being forced into alignment.

We pment a model lo describe certain aspens of the multiphoton multielectron dissociative ionization of simple molecules m tnteme laser fields. We obtain fragment ion dissociation energies and appearance Intensities that are in good agreement with experiment.The multielectron dissociative ionization of diatomic and triatomic molecules in intense laser fields (focal intensities W cm-*) is not understood. Recent experiments indicate that all fragment ions emerge with dissociation energies which are specific fiactions (-70% for 12. -45% for NI, etc) of the Coulomb explosion energy, regardless of their charge state (see, for example, Frasinski et , Hatherly et a[ 1994, Schmidt er al 1994. This Coulomb explosion energy, E,, is given, in the diatomic case, by the expression E, = Ql Q2/RP (in atomic units), where Q, and Q 2 are the atomic ion charges and Re is the equilibrium internuclear separation of the neutral molecule. One might argue that the reduction in Coulomb energy is caused by different screening at each ionization stage (a reduction in Ql and Q2) at Re. A simpler explanation is that the molecule (molecular ion) relaxes to a critical distance, R,, where it stabilizes and subsequently ionizes, with little or no change in inter-ion separation for the different ionization stages. For example, for I1 (Re = 2.67 A) a value of R, of 3.8 %, (= 2.67/70%) would explain the observations. We have developed a simple model which allows us to predict the value of R, for any ionization stage of a diatomic (or linear triatomic) molecule. The only data required by the model are the atomic ionization potentials. We find, quite surprisingly, that the value of Rc is virtually independent of the stage of ionization of the molecule and that using this value of R, reproduces the experimental values for the dissociation energies of the various fragment ion pairs. Moreover, the model is able to predict appearance (threshold) intensities for 12 that agree well with the experimental data of Strickland er al (1992).Consider the molecular ion I: dissociating in a laser electric field of steadily rising amplitude, with the molecular axis lying along the field direction. We focus our attention on the outer electron, and combine the remaining electrons and nuclei into two point-like atomic ions. Initially the outer electron can move freely in the double well, but as the ions move apart, the inner barrier rises and at a certain, critical internuclear distance it starts to impede the motion of the electron: the electron can no longer follow the field and the system makes a transition to a non-dissociative state (see later discussion). The situation that

H2 molecules were ionized by Ti:sapphire (45 fs, 800 nm) and Nd-doped yttrium aluminum garnet lasers (6 ns, 1064 nm). The relative populations of the vibrational levels of the H+2 ions were determined and found to be concentrated in the lowest vibrational levels. Tunneling ionization calculations with exact field-modified potential curves reproduce the experimental results. The reason for the departure from conventional Franck-Condon-like distributions is the rapid variation of the ionization rate with internuclear distance.

Phase control of the spatial asymmetry of the dissociative ionization channel, H+H + , of molecular hydrogen has been achieved by superposing interferometrically sub-100 fs, Ti:sapphire laser pulses at the fundamental and second harmonic frequency. The counterintuitive asymmetry previously observed with longer, less intense Nd:YLF laser pulses and HD molecules has been reproduced. However, phase modulation of total ionization and dissociation rates was not discernible. In order to further understand the process, a comprehensive study of appearance intensities, kinetic energy releases and angular distributions has been undertaken at each frequency.

A refinement to the field-ionization, Coulomb explosion model of diatomic molecules in intense laser fields is described, which predicts classical appearance intensities for molecular ions as a function of internuclear separation. The ionization process is found to be optimized at the critical internuclear distance, , which is virtually the same for all fragmentation channels. The appearance intensities are coupled with classical trajectory calculations to produce fragmentation energy releases that are in good agreement with experiment, in the case of at a wavelength of 750 nm and for a range of pulse lengths from 55 to 400 fs. The model also predicts that charge transfer will lead to the observation of the charge-asymmetric channel at pulse lengths below about 200 fs; this prediction is confirmed by experiment. At 30 ps, the observation of substantial post-dissociative ionization is explained by a reduction in the dissociation rate of the ions at 1064 nm. Data on using 55 and 400 fs pulses at 750 nm show the effect of competition between ionization and dissociation. An explanation is given for the contrasting ion time-of-flight (TOF) spectra at visible and ultraviolet wavelengths. Finally, in the case of in short, intense laser fields, the ion TOF spectra are explained without recourse to vibrational trapping or bond hardening in .

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