Probing Molecular Dynamics at Attosecond Resolution with Femtosecond Laser Pulses

Abstract

The kinetic energy distribution of D + ions resulting from the interaction of a femtosecond laser pulse with D2 molecules is calculated based on the rescattering model. From analyzing the molecular dynamics, it is shown that the recollision time between the ionized electron and the D + 2 ion can be read from the D + kinetic energy peaks to attosecond accuracy. We further suggest that more precise reading of the clock can be achieved by using shorter fs laser pulses (about 15fs).PACS numbers: 34.50. Rk, 31.70.Hq, 95.55.Sh Human experience shows that new areas of science and technology open up with the ability to make measurements at increasingly shorter time regime. With the advent of femtosecond (fs) lasers, femtochemistry became possible where chemical reaction dynamics can be probed at the atomic scale [1]. Clearly, fs lasers cannot be used directly to probe electron dynamics which is in the attosecond (as) regime. While a substantial effort is being dedicated to developing single attosecond pulses [2,3,4,5], presently few laboratories have such lasers available.An ingenious suggestion for performing measurements at attosecond resolution with fs lasers was proposed by Corkum and his group recently. Their results were reported in two recent publications, here to be called I [6] and II [7], respectively. In their experiment, a 40 fs pulse, with mean wavelength ranging from 800 nm to 1850 nm, and peak intensity of about 1.5 × 10 14 W/cm 2 , was used to ionize a D 2 molecule to produce D + ion. It was assumed that D 2 was first ionized near the peak of the laser pulse to create a correlated electronic and nuclear wave packet. Within a single optical cycle, the electron was driven back to collide with D + 2 and to excite it to the excited σ u electronic state which subsequently dissociated to produce D + . The kinetic energy of the D + ion reflects the internuclear distance, as well as the time when the rescattering occurs. With proper laser intensity, both the initial ionization and the rescattering are found to occur at time interval of far less than one optical cycle, thus providing attosecond temporal resolution, irrespective of the femtosecond pulse duration of the laser. In II, by changing the wavelength of the fs laser, they concluded that the dissociation dynamics of D + 2 can be used as a molecular clock and the clock can be read with attosecond resolution.To read the molecular clock accurately, the rescatter- * Contact: xmtong@phys.ksu.edu ing mechanism which leads to the measurable D + kinetic energy distribution has to be understood in details. In this Letter we report the main conclusion of our careful analysis of the rescattering mechanism. In contradiction to I and II, our analysis shows that the D + ions are not produced by the dissociation of the excited D + produced from Coulomb explosion vs. from dissociation was determined. Our analysis also shows that the dominant peak of the D + kinetic energy distribution is from the third return of the rescattering process, rather than from the first retu...

Attosecond Temporal Gating with Elliptically Polarized Light

“…We show that gating can be applied directly to the collision process enabling attosecond pulses to be measured as they are produced. This approach can be extended to probe molecular dynamics [7,8], attosecond multielectron [9,10], or nuclear [11] dynamics with much greater resolution.…”

“…We show that gating can be applied directly to the collision process enabling attosecond pulses to be measured as they are produced. This approach can be extended to probe molecular dynamics [7,8], attosecond multielectron [9,10], or nuclear [11] dynamics with much greater resolution.Recollision occurs when an intense laser field removes an electron from its parent atom (often via tunnel ionization). As the free electron wave packet oscillates in the laser field, it can elastically or inelastically scatter with its parent atom, producing excitation, multiple ionization, or extreme ultraviolet (XUV) radiation.…”

“…This recollision occurs on the sub-laser-cycle time scale. Attosecond dynamics experiments can use the attosecond optical pulses [6] or the attosecond electron-ion interaction [7,8] for measuring dynamics. We show that gating can be applied directly to the collision process enabling attosecond pulses to be measured as they are produced.…”

Molecular ionization and dissociation using synchrotron radiation and ultrashort laser pulses

“…For this wavelength, ionization requires the absorption of many photons, which makes the process highly non perturbative even at relatively low intensities (e.g., 10 12 W/cm 2 ). Thus most theoretical approaches used to interpret these experiments are based on high-field models, like the ADK model [84,93], simplified Floquet approaches [94], or reduced time-dependent calculations [95]. In contrast, the production of fs and sub-fs, vuv and xuv laser pulses by means of high-order harmonic generation [96][97][98] or free electron lasers [99][100][101] has opened up the way to investigate simpler two-and three-photon ionization processes in H + 2 and H 2 by solving the TDSE.…”

Photon–ion collisions and molecular clocks

“…In recent years two additional double ionization processes have been identified and modelled: rescattering ionization [15][16][17][18][19][20][21] and sequential ionization [22,23]. Figure 6 shows a schematic of these three processes, roughly ordered according to the intensity region in which each is expected to dominate.…”