Multiple ionization of N 2 in intense, linearly and circularly polarized light fields S. Banerjee, G. Ravindra Kumar, and D. Mathur Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400 005, India Received 20 November 1998; revised manuscript received 8 February 1999 There are significant differences in the multielectron dissociative ionization MEDI of N 2 by 100-fs-long laser pulses intensity 10 15 W cm -2 ) using linearly and circularly polarized light, with substantial suppression of ionization in the latter case. Enhanced ionization occurs in both instances at an internuclear distance (r) of 2.2 Å, with an increased propensity for MEDI at larger r values with circularly polarized light. S1050-29479950107-8 PACS numbers: 33.80.Rv, 33.80.Wz, 42.50.Hz Much progress has been made in recent years in gaining insights into a host of counterintuitive phenomena that arise in interactions of intense light fields with matter such as above-threshold ionization ATI and dissociation, high har- monic generation HHG, and stabilization 1. The strongly nonperturbative physics that governs such phenomena has made theoretical analysis difficult. Most of the effort has been on the application of one-dimensional numerical tech- niques directed towards investigations with linearly polar- ized light. Light of arbitrary polarization requires the use of at least two spatial dimensions. Some progress has been re- cently reported in studies of laser-atom interactions with light of arbitrary polarization 2. In these theoretical studies circular polarization is shown to enhance ionization of a model atom at all but the lowest laser intensities ( 10 13 W cm -2 at 526 nm. On the other hand, in other studies of intense field atomic phenomena, circular polariza- tion has usually been observed to result in the suppression of ionization as in ATI and of light emission HHG3. In atomic ionization that occurs in the low-intensity perturba- tive multiphoton regime, circular polarization can either en- hance ionization as in the case of Cs 4 or suppress it 5. The extension of such investigations to polarization- dependent molecular dynamics has hitherto remained virgin territory, and is the subject of the present study. Intense-field molecular dynamics with linearly and circularly polarized light is a subject that is both interesting and of importance because of the richness that is added by facets that are pecu- liar to molecules, such as enhanced ionization, competition between ionization and dissociation, bond softening/ hardening, and spatial reorientation effects. It remains to be investigated how circular polarization affects the efficacy of existing one-dimensional models that predict phenomena such as enhanced ionization 6. Furthermore, circularly po- larized light cannot be merely treated as a combination of two perpendicular, linearly polarized components. In other words, the dynamics resulting from irradiation of molecules by circularly polarized light is not expected to be a linear combination of the dynamical effects due to linearly polar- ized light aligned parallel and perpendicular to the molecular symmetry axis. Moreover, circularly polarized light imparts angular momentum to the atom/molecule, whereas linearly polarized light does not. How this might affect molecular dynamics in intense laser fields is an issue that has not, to our knowledge, been addressed. We report here results of experi- ments on the multielectron dissociative ionization MEDI of a simple diatomic molecule, N 2 , in intense, 100-fs-duration light fields of intensity 10 15 W cm -2 , wavelength 806 nm using coincidence time-of-flight TOF spectrometry. The morphology of our data reveals significant differences in the ionization dynamics using light that is linearly polarized and that obtained with circularly polarized light of the same in- tensity 7. Care was taken to also make measurements with circularly polarized light at intensities that yielded the same electric field as in the case of linear polarization. The differ- ences that are observed manifest themselves in significantly reduced total ion yields in the latter case, along with en- hancement of lower-energy components in the kinetic- energy distribution functions measured for products of MEDI. In our experiments, multiple ionization occurs in the tunneling regime Keldysh parameter, 0.14. The femtosecond laser used in the current experiments is a chirped pulse amplification, titanium-sapphire system com- prising an oscillator, pulse-stretcher, regenerative amplifier, multipass amplifier, and a grating pulse-compressor. The peak energy output is 50–55 mJ per pulse, with a pulse du- ration of 100 fs at a repetition rate of 10 Hz. The resulting unfocused output power is 0.5 TW. The laser pulses are temporally, spatially, and spectrally characterized at the out- put and at various intermediate stages. Because of dispersion in optical elements in the beam path, the final pulse duration in the interaction region is 150 fs. Multiple ionization was studied using a linear, two-field, time-of-flight spectrometer in a large 85-cm-diam, stainless-steel, ultra-high-vacuum chamber capable of background pressures of 5 10 -11 Torr. The spectrometer was designed such that only a subset of the total laser focal volume was sampled, thereby ensuring that only a limited range of laser intensities was accessed in the course of the measurement of mass spectra 8. Ions were detected using a channel electron multiplier CEM operating in the particle counting mode. The CEM output was taken to a 100-MHz digital oscilloscope after amplification by a fast preamplifier and linked to a laboratory computer by a fast data bus. Operating pressures with gas load were maintained low enough in the range 6 10 -9 –6 10 -8 Torr to ensure that space-charge effects were negligible. The polarization state was varied by use of a half-wave or quarter-wave plate; on-line monitoring of the laser intensity ensured a constant value in the course of mea- surements with different polarizations. RAPID COMMUNICATIONS PHYSICAL REVIEW A JULY 1999 VOLUME 60, NUMBER 1 PRA 60 1050-2947/99/601/254/$15.00 R25 ©1999 The American Physical Society