Interaction of ultrashort intense laser pulses with molecular ion beams

I. Ben-Itzhak, P.Q. Wang, A.M. Sayler, B. Gaire, N.G. Johnson, M. Leonard, E. Parke, K.D. Carnes, Vladimir Roudnev, B.D. Esry
J. R. Macdonald Laboratory, Department of Physics, Kansas State University, Manhattan, KS, USA

Laser induced dissociation and ionization of H₂⁺ have been experimentally studied using intense (10¹³-10¹⁵ W/cm²) short (8-135 fs) laser pulses at 790 and 395 nm. Both fragments from the dissociation of the vibrationally excited H₂⁺ beam are measured in coincidence by a 3-dimensional momentum imaging system event by event.

The kinetic energy release distribution following ionization of H₂⁺ by 45 fs, 10¹⁴-10¹⁵ W/cm² laser pulses, i.e. in the tunneling regime, was measured to be broad and structureless. Its peak shifts toward higher energies as the laser intensity is increased indicating that ionization shifts to smaller internuclear distances. In contrast, a surprising structure is observed near the ionization threshold, which we call above threshold Coulomb explosion [1], defined as resonant multiphoton ionization of H₂⁺ during its dissociation along the bond softening (BS), bond hardening (BH) or above threshold dissociation (ATD) pathways. The angular distributions of the two H⁺ fragments are strongly peaked along the laser polarization, and the angular distribution is described well by [cos²θ]ⁿ, where n is the number of photons predicted for the resonant multiphoton process near the ionization threshold. Our data indicates that n varies as expected with the laser wavelength.

The dissociation results change dramatically with decreasing pulse width, in contrast to the reported trend for longer pulses [2]. At similar peak intensities, determined by using an intensity-difference spectrum method [3], bond-softening is found to be the main feature in 135 fs pulses, while in 45 fs pulses it is a minor process having a low kinetic energy release and a very narrow angular distribution. Above threshold dissociation is dominant in 45 fs pulses whose durations are approaching the vibrational period of the molecule [4]. The measurements with 135 fs pulses at ~10¹⁴ W/cm² reveal a strong correlation between the angular distribution and the kinetic energy released upon dissociation [5]. This correlation is in contrast to the cos²θ distribution of photo-dissociation in the weak-field limit. Bond softening is aligned along the laser polarization much more than weak-field photo-dissociation. In contrast, bond hardening results in wider angular distributions than photo-dissociation.

Preliminary results with few cycle (8 fs) pulses indicate similar trends as discussed above. Attempts to determine the carrier-envelope phase (CEP) of each pulse are underway in order to measure the predicted dependence of the angular distribution of dissociation to H⁺ + D and H + D⁺ on the CEP [6].

References:

1. B.D. Esry, A.M. Sayler, P.Q. Wang, K.D. Carnes, and I. Ben-Itzhak, Phys. Rev. Lett. - submitted.
2. K. Sändig, H. Figger, and T.W. Hänsch, Phys. Rev. Lett. 85, 4876 (2000).
3. P.Q. Wang, A.M. Sayler, K.D. Carnes, B.D. Esry, and I. Ben-Itzhak, Opt. Lett. 30, 664 (2005).
4. I. Ben-Itzhak, P.Q. Wang, J.F. Xia, A.M. Sayler, M.A. Smith, K.D. Carnes, and B.D. Esry, Phys. Rev. Lett. 95, 073002 (2005).
5. P.Q. Wang, A.M. Sayler, K.D. Carnes, J.F. Xia, M.A. Smith, B.D. Esry and I. Ben-Itzhak, J. Phys. B 38, L251 (2005).
6. Vladimir Roudnev, B.D. Esry, and I. Ben-Itzhak, Phys. Rev. Lett. 93, 163601 (2004).

Acknowledgments: Thanks to B. Shan and Z. Chang for providing the intense laser beams, and C. Fehrenbach for the ECR beam. This work was supported by the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy.

Submitted to LPHYS'06, July 2006 in Lausanne, Switzerland.

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