Conditions for extracting photoionization cross sections from laser-induced high-order-harmonic spectra

Abstract

We show that photoionization cross sections of atoms can be accurately extracted from the high-order-harmonic spectra generated by intense infrared lasers only if the degree of ionization in the gas medium is small, implying that for this purpose the high-order-harmonic generation spectra should be taken at low gas pressure and at low laser intensity experimentally. In a recent paper, Shiner et al.[1] used the so-called highorder-harmonic spectroscopy to probe collective multielectron dynamics in Xe. By comparing the measured high-orderharmonic generation (HHG) spectra of Kr and Xe atoms generated by intense 1.8-μm lasers and assuming that the photoionization cross section (PICS) of Kr is known, they deduced the differential PICS of Xe from the laser-generated HHG spectra. The deduced PICS reveals the well-known strong peak around 100 eV (photon energy), in good agreement with the 5p partial PICS that has been previously measured with synchrotron radiation light sources [2]. In the parlance of photoionization, this enhancement is caused by the so-called intershell coupling [3] with photoionization from the 4d shell of Xe, which is a specific form of the many-electron correlation effect [4]. To employ the procedure used by Shiner et al.[1], a number of assumptions have to be made. In this Brief Report, we examine the validity of these assumptions.The conceptual connection between HHG and PI is built on the well-known three-step model [5,6] for HHG. In the third step, photorecombination is the inverse of photoionization, and the photorecombination cross section (PRCS) is trivially related to the PICS via the principle of detailed balance. Recently, the three-step model has been cast in a more rigorous form, in the quantitative rescattering (QRS) theory [7][8][9]. According to QRS and some relevant works [10], the HHG by an atom or molecule is related to the PRCS σ r bywhere S(ω) is the HHG power spectrum and w(ω) is the so-called returning electron wave packet. In fact, Eq. (1) has been established at the level of complex amplitudes; thus each quantity in the equation has a magnitude and a phase [8,9]. Unlike PICS from conventional measurements, the phase of each harmonic can be determined experimentally [11], from which the phase of photorecombination transition dipole moment can be obtained. The validity of Eq. (1) has been established in our previous works [12]. Since the harmonic spectrum extends over a broadband, the electron wave packet also covers a broadband where the energies of the electrons and photons are related byhω = I P + E, where ω is the angular frequency of the photon, I P is the ionization energy of the atom, and E is the "incident" electron energy. According to QRS, σ r is directly related to the laser-free PICS and does not depend on the properties of the laser. On the other hand,

High-order harmonic generation spectroscopy by recolliding electron caustics

“…A retrieval of the macroscopic response of the target often requires the full knowledge of the sample and pulse properties, in addition to time-consuming calculations. As a consequence, QRS can only be efficiently applied at low pressures and laser intensities, in order to avoid saturation and plasma defocusing [14]. Moreover, QRS cannot reproduce the influence of the ionic state on the propagation of the electron as well as the evolution of the ionic state during this propagation.…”

Effects of pressure and gas-jet thickness on the generation of attosecond pulse

“…Recently, we successfully simulated the macroscopic HHG spectra of Ar, Ne, Xe and Kr. [17,18] By solving the proper Maxwell wave equations, the simulations showed a good agreement with the experimental measurements. In this paper, our goal is to investigate how the pressure and the thickness of the gas jet affect the generation of AP.…”

Control of soft X-ray high harmonic spectrum by using two-color laser pulses

“…Thus the "minimum" structure in the PRCS would lead to suppression of harmonics in the single-atom spectrum. Such minimum could be preserved after macroscopic propagation in a gaseous medium under certain conditions [23]. A well-known example is the "Cooper minimum" in Ar observed in high harmonic spectra [24][25][26][27][28][29][30].…”