Nonsequential Two-Photon Double Ionization of Atoms: Identifying the Mechanism

Abstract

We develop an approximate model for the process of direct (nonsequential) two-photon double ionization of atoms. Employing the model, we calculate (generalized) total cross sections as well as energy-resolved differential cross sections of helium for photon energies ranging from 39 to 54 eV. A comparison with results of ab initio calculations reveals that the agreement is at a quantitative level. We thus demonstrate that this complex ionization process is fully described by the simple model, providing insight into the underlying physical mechanism. Finally, we use the model to calculate generalized cross sections for the two-photon double ionization of neon in the nonsequential regime.PACS numbers: 32.80. Rm, 32.80.Fb, 42.50.Hz Correlated dynamical processes in nature poses unique challenges to experiments and theory. A prime example of this is the double ionization of helium by one-photon impact, which has been studied for more than 40 years. However, it is only during the last 15 years or so, that advances in theory, modeling and experiment have enabled scientists to gain a deeper insight into the role of electron correlations in this ionization process [1][2][3][4][5]. The corresponding problem of two-photon double ionization of helium, in the photon energy interval between 39.4 and 54.4 eV, is an outstanding quantum mechanical problem that has been, and still is, subject to intense research worldwide, both theoretically [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] and experimentally, employing state-of-the-art high-order harmonic [20][21][22] and free-electron (FEL) light sources [23,24]. Despite all the interest and efforts that have been put into this research, major fundamental issues remain unresolved. What characterizes this particular three-body breakup process is that the electron correlation is a prerequisite for the process to occur, i.e., it depends upon the exchange of energy between the outgoing electrons, and as such it represents a clear departure from an independent-particle picture.In this Letter, we present a novel approximate model for the direct or nonsequential two-photon double ionization process in helium, sketched in Fig. 1 (a). We show that the simple model predicts the essential features of the process, even at a quantitative level, which is quite surprising given the very high complexity of the problem. In particular, we find very good agreement between the model predictions and the results obtained by solving the time-dependent Schrödinger equation from first principles, regarding (generalized) total cross sections as well as energy-resolved differential cross sections for the process. The proposed model may be generalized to account for direct double ionization processes in multi-electron atoms. We demonstrate this by calculating the generalized cross section for nonsequential two-photon double ionization of neon.Few-photon multiple ionization of noble gases beyond helium have been studied experimentally in some detail [23][24][25][26], but to the best of our knowled...

Theory of two-photon double ionization of helium at the sequential threshold

“…The model leads to a simple expression of the differential TPDI cross section (since the TPDI probability varies linearly with T within LOPT it is appropriate to calculate cross sections); see Eq. (8) in [3]. The results agree qualitatively and quantitatively with other calculations, at least for ω > 1.84 (50 eV), where it is not critical for the electrons to interact strongly in order to overcome the attraction of the nucleus [2].…”

“…In addition to these discrepancies between theoretical results, there is a new issue, namely, the sharp rise of the TPDI cross section around 2 a.u. [3][4][5][6][7][8], that is the subject of ongoing debates [9]. This brings our attention to TPDI in the threshold region of the sequential process, i.e., at photon energies close to 2 a.u.…”

Nonsequential double ionization of the hydride ion by two-photon absorption

“…3. Recently, it was predicted that the anti-U shape could also appear in neon at lower photon energies [32], and it has furthermore been suggested that the energy spectrum of helium has a negative concavity near the line in the figure shows the corresponding SDCS obtained for helium at the photon energy 40 eV, close to the lower threshold in helium (39.4 eV), and indeed a slightly negative concavity is observed. The SDCS result for helium (40 eV) was derived from a pulse 7 fs in duration and extracted by projecting onto Z = 2 Coulomb waves.…”

“…As a rather complete understanding of the process of one-photon double ionization of helium is emerging [17,18] the related process of nonsequential two-photon double ionization (TPDI) is currently receiving considerable theoretical [12,[19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] and experimental [36][37][38][39][40][41] attention. Recently, TPDI of H 2 by 30-eV photons was studied theoretically [42][43][44], and clear discrepancies in the calculated tripledifferential cross sections were identified.…”

Recoil-ion-momentum spectrum for few-photon double ionization of helium

“…The one-photon and two-photon double ionization of helium has attracted remarkable attention [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] since these processes provide a wonderful chance to investigate the fundamental two-electron correlation, which is related to the long-standing three-body Coulomb problem. Direct coincidence measurements on the fully differential two-electron momentum distributions can provide detailed information about the process of double ionization.…”