Autoionizing Decay of H2 Doubly Excited States by Using Xuv-Pump–Infrared-Probe Schemes with Trains of Attosecond Pulses

PHYSICAL REVIEW A 85, 063414 (2012)

Autoionizing decay of H2 doubly excited states by using xuv-pump–infrared-probe schemes with trains of attosecond pulses

R. E. F. Silva,1 P. Riviere,` 1,* and F. Mart´ın1,2 1Departamento de Qu´ımica, Modulo´ 13, Universidad Autonoma´ de Madrid, 28049 Madrid, Spain 2Instituto Madrileno˜ de Estudios Avanzados en Nanociencia, Cantoblanco, 28049 Madrid, Spain (Received 24 April 2012; published 22 June 2012)

We present a theoretical study of H2 ionization by a pump-probe scheme consisting on an attosecond pulse train (APT) and a near-infrared (IR) pulse. We focus on the autoionization dynamics of the ﬁrst series of resonant states 1 + of the molecule, the Q1 doubly excited states. The APT central frequency is tuned to populate the u resonant states. The trace of autoionization is clearly visible in the two-dimensional (2D) proton-electron coincidence spectra and in the proton kinetic energy spectra. The dynamics of the autoionization process is clearly visible in the movie obtained by plotting the 2D spectrum as a function of the time delay between the APT and IR pulses.

An analysis of the ﬁnal symmetries g and u allows us to track the origin of the different structures.

DOI: 10.1103/PhysRevA.85.063414 PACS number(s): 33.80.Eh, 33.20.Xx, 42.65.Re

I. INTRODUCTION ionization. The DES states can also dissociate into two neutral (H + H) or ionic (H+ + H−) fragments. Ultrashort laser pulses are an excellent tool to study the So far, single attosecond pulses (SAP) have been used dynamics of molecular photoionization. Combined with kine- to study autoionization in H both in xuv-pump–xuv-probe matically complete experiments, in which the momentum of all 2 setups [19] and in combination with IR fields [20]. The ejected electrons and atomic ions is determined in coincidence, latter scheme has been successfully used to probe electron they have made possible to obtain detailed information of the wave packet dynamics in atoms [21], and its acceleration photoionization process [1–8]. These kinds of experiments (streaking) by intense infrared (IR) fields [22]. However, open the door to unravel the dynamics of autoionization, a SAPs have broad spectral bandwidths that populate a large process triggered by electron correlation. Autoionization may number of states and, consequently, they are not very selective. occur when two or more valence electrons are excited and one An alternative are attosecond pulse trains (APT), formed by of them is spontaneously ejected in the continuum, or when consecutive attosecond pulses. APT are typically created from one inner-shell electron is missing, so an electron from an high harmonic generation, and they combine two advantages. upper shell occupies the hole and another electron is ejected First, they are easier to produce experimentally. Second, their (Auger effect). This laser-assisted photoemission process is spectral selectivity is much higher: the energy spectrum of an now widely used to track fast electron dynamics in atoms, and APT is not a broad distribution around the frequency of the to characterize x-ray pulses [9–13]. pulse, such as in a SAP, but a series of spikes at odd multiples In molecules, autoionization lifetimes are typically of the of the generating laser frequency. Therefore, the number of same order of magnitude as the molecular vibrational period populated states is greatly reduced with respect to the SAP. or the time for molecular dissociation. Thus the nuclei have Such APT-pump–IR-probe schemes have recently been used enough time to move outside the Franck-Condon (FC) region to study IR-assisted ionization of atoms [23,24], as well as to before the electron is ejected. Therefore, a correct theoretical induce reconstruction of attosecond beatings by interference of description requires not only that electron correlation is well two-photon transitions (RABITT) in H2 photoionization [25]. described, but also that nuclear motion is included. Further- However, a systematic study of the dynamics of autoioniza- more, autoionization and direct ionization may interfere, so tion in H is still to be done. In this work we use a pump-probe a fully quantum mechanical treatment of both nuclear and 2 scheme to study the dynamics of the Q1 doubly excited states electronic motion is required. Hence describing autoionization 1 + of the H2 molecule with u symmetry. The pump field in molecules is in general a formidable challenge. is an APT whose most intense harmonic is tuned with the For these reasons, a complete description of the autoion- energy difference between the ground state of the molecule 1 + ization process in molecules is only possible for the simplest and the first Q1 u resonant state. The probe is a near-IR molecular system, H2 (or D2), for which a theoretical method pulse used to track the autoionization dynamics. Both fields that fulfills the previous requirements has been recently are linearly polarized in the direction of the internuclear axis. proposed and adapted to treat autoionization [14–16]. This In our spectral method we can substract any state we want is the method that will be used in the present work. In the from the calculations, so in order to study the effect of the 1 + H2 molecule, doubly excited states (DES) are embedded in Q1 u resonances, we compare the results for both the full and coupled to the electronic continuum through electron calculation and one in which we have removed those states correlation [17,18], and can thus contribute to nondissociative in the calculation. We also split the results into different final ∗∗ + − ∗∗ + − symmetries. (H → H2 + e ) or dissociative (H → H + H + e ) The paper is organized as follows. In Sec. II we de- scribe in detail the theoretical method. Results are shown in Sec. III, focusing on photoelectron-photoion coincidence *[email protected] spectra (Sec. III A), photoelectron spectra (Sec. III B), and

1050-2947/2012/85(6)/063414(7)063414-1 ©2012 American Physical Society R. E. F. SILVA, P. RIVIERE,` AND F. MARTIN´ PHYSICAL REVIEW A 85, 063414 (2012) proton kinetic energy release spectra (Sec. III C). Finally, Once the electronic structure has been obtained in a some conclusions are drawn in Sec. IV. Atomic units are used given grid of internuclear distances, we solve the nuclear throughout unless otherwise specified. one-dimensional Schrodinger¨ equation, ∇2 − R + E (R) χ (R) = W χ (R) , (7) II. THEORETICAL FRAMEWORK 2μ x νx νx νx Since the method has been explained in detail in Refs. where x stands for a bound, continuum, or resonant state of H2, [14–16], here we briefly summarize the basic ingredients. We and Wνx is the vibronic energy associated with the x electronic solve the time-dependent Schrodinger¨ equation (TDSE) state. The total wave function is thus written ∂ − = iWνn t Hˆ (r,R) + Vˆ (t) − i (r,R,t) = 0, (1) (r,R,t) Cnνn (t) n (r; R) χνn (R)e 0 ∂t n νn where r represents the electronic coordinates r and r , and R − 1 2 + iWνr t Crνr (t) r (r; R) χνr (R)e is the internuclear distance. Hˆ0 is the field-free nonrelativistic r νr Hamiltonian of the H2 molecule, εl εl −iWν t + dε C (t) (r; R) χν (R)e α , Hˆ0(r,R) = Tˆ (R) + Hêl(r,R), (2) α,να α α α,l να ˆ =−∇ˆ 2 where T (R) R/2μ is the relative kinetic energy of (8) the nuclei, μ is the reduced mass, and Hêl(r,R)isthe electronic Hamiltonian which includes the 1/R repulsion term. where the three terms represent the bound states, the resonant We neglect mass polarization terms, relativistic effects, and states, and the continuum states, respectively. Inserting Eq. (8) nonadiabatic couplings. in Eq. (1) and projecting onto the basis of vibronic states leads To deal with autoionization, we use the Feshbach for- to a set of coupled differential equations where the states with malism [18]. In this framework, we define two orthogonal different symmetry are coupled by the laser potential, and complementary subspaces P and Q = 1 − P , with associated the continuum and resonant states with the same symmetry ˆ ˆ ˆ ˆ ˆ ˆ projectors Pˆ and Qˆ , and expand the total wave function are coupled by QHelP and P HelQ terms. The TDSE is = (r,R,t) in the basis of molecular eigenstates associated with then integrated up to tmax TAI, where TAI 1/ is the each subspace. Since these states are not eigenstates of the maximum autoionization time (corresponding to the minimum autoionization width of all populated DES). The (r,R) molecular Hamiltonian, they will be coupled in the TDSE. kνk More specifically, the time-dependent wave function (r,R,t) basis states are eigenstates of the field-free Hamiltonian at =∞ is expanded in a basis of vibronic stationary states, which t , so the projection of the total wave function onto these are written as products of an electronic wave function and a states provides amplitudes that can be written in terms of the vibrational or dissociative wave function [26]. The expansion Ci expansion coefficients. includes three kinds of electronic wave functions, namely The interaction potential between the laser field and the H2 ˆ bound states, resonant states, and continuum states, calculated molecule, V (t), is obtained within the dipole approximation, in a dense enough grid of internuclear distances [15]. The and using the velocity gauge it can be expressed as ˆ = + · ˆ two former are obtained from configuration interaction cal- V (t) (pˆ 1 pˆ 2) A(t), (9) 2 culations, while the latter is obtained from L close-coupling ˆ calculations performed within a box [27,28]. where pˆ i is the momentum operator of electron i and A(t) The bound states of the H molecule are eigenfunctions is the operator of the vector potential of the electromagnetic n 2 field, which is described classically. of the electronic Hamiltonian, 1 + To study the Q1 u resonances, we use an APT-pump–IR- Hêl (r,R) n (r) = En (R) n (r) , (3) probe setup, with linearly polarized light whose polarization direction is parallel to the internuclear axis. Thus only states with eigenenergy En. The resonances associated with the Q + + of g and u symmetry will be populated. The APT is subspace are eigenfunctions of the projected Hamiltonian in formed by a train of four xuv pulses with FWHM = 200 as Q subspace, = and frequency 17ω, where ω 1.605 eV is the frequency Qˆ HêlQˆ − Er (R) r (r; R) = 0(4)of the generating IR field. The delay between the pulses in the train is T/2, where T is the IR period, which leads and the nonresonant states associated with the electronic to an energy spectrum formed by odd harmonics of the continuum in the P subspace are the solutions of the projected generating frequency. This originates an energy spectrum with Schrodinger¨ equation, two principal harmonics at frequencies 17ω and 19ω (see [PˆHˆ Pˆ − E(R)]εl(r; R) = 0, (5) Fig. 1), that we will denote H17 and H19, and the less intense el α harmonics H13, H15, H21, and H23. As can be seen in the 1 + E(R) − E (R) + ε = 0. (6) figure, all parameters are chosen such that the Q1 u series α of DESs is significantly populated. The intensity is 109 W/cm2 Here α denotes the full set of quantum numbers for the for the two central pulses and 0.35 × 109 W/cm2 for the + electronic state of the molecular ion H2 with a Born- external pulses, which corresponds to an APT envelope with 9 2 Oppenheimer energy Eα (R), and ε and l are the energy and FWHM = 3 fs and I = 1.14 × 10 W/cm . The IR probe angular momentum of the ejected electron. (ω = 1.605 eV), which can also be used to generate the above

063414-2 AUTOIONIZING DECAY OF H2 DOUBLY EXCITED ... PHYSICAL REVIEW A 85, 063414 (2012)

σ -10 u Σ Q Σ Q1 u 1 g σ

-20 1Σ + (X g )

-30 FIG. 2. (Color online) Doubly differential photoelectron-proton kinetic energy spectra for a SAP (a), and APT + IR at three different 123456 time delays: t =−T (b), 0 (c), and T/2 (d), where T is the IR R(a.u.) period. The dashed lines represent the energy conservation lines for the different harmonics (see text). The uppermost line corresponds to FIG. 1. (Color online) Born-Oppenheimer potential energy H21. All probabilities are multiplied by 106. + curves of the H2 and H2 molecules. The energy spectrum of the APT is depicted in orange. The gray zone corresponds to ionization, energy (EKE) and the proton kinetic energy (PKE), either and the Franck-Condon region lies between the vertical dashed lines. measured in coincidence or irrespective of each other. One The black vertical arrow represents a vertical transition from the H2 of the advantages of using a spectral method is that we can 1 + ground state to the lowest Q1 u resonance at the H2 equilibrium select the states involved in the calculation as well as easily distance (R = 1.4 a.u.). identify the final ionization channel. To unravel the role of the resonances we have performed three sets of calculations: with APT, is three cycles long, has a sinus-square envelope, and 1 + all states included, without the Q1 u states, and without the an intensity of 1012 W/cm2. To probe the dynamics of the 1 + Q1 g states. Our calculations show that, in all cases, the resonant state, we perform a scan in delays between the APT 1 + effect of removing the g resonances, which need at least pump and the IR probe. The delay t is defined with respect to two photons to be populated, is almost negligible. Therefore, the center of their corresponding envelopes, and t > 0 means = we will mainly focus on the first two sets. that the xuv APT comes before the IR pulse. For t 0, the We note that H single ionization leads predominantly to xuv pulses are located at maxima and minima of the IR field. 2 vibrationally bound states in the 1sσg electronic state of the The Born-Oppenheimer curves for the H2 molecule are + + − H2 molecular ion [H2 → H2 (1sσg,v) + e ]; see Fig. 1. shown in Fig. 1, together with the APT spectrum. The figure Therefore, most electrons in the EKE distributions come from shows the H2 ground state and the two first electronic states of + this nondissociative channel. Dissociative ionization is about the H2 ion, 1sσg and 2pσu. The first series of resonances, + + two orders of magnitude smaller and is observed in both the Q1, is shown for both the g (in red) and u (in blue) 1 + 2pσu and 1sσg channels. However, as Q1 u resonances symmetries of the molecule. Note that all potential energy mainly decay into the dissociative part of the ionization curves for the DES are repulsive, i.e., these resonant states + continuum, they will be more visible in the PKE spectrum can either dissociate on two neutrals, or decay into the H2 than in the EKE one. An even more complete dynamical continuum. picture can be obtained by analyzing the two-dimensional (2D) Within the dipole approximation and for linearly polarized coincidence photoelectron-proton kinetic energy spectrum. light, the selection rules impose that the total symmetry of Autoionization of the Q 1+ states will lead to either H + the molecule changes parity after absorption of one photon. 1 u 2 in the 1sσg state plus an ejected electron with odd angular Therefore, if the initial state of the system is the ground + 1 + 1 + momentum or to H2 in the 2pσu state plus an ejected electron rovibrational state of H2, X g , then the Q1 u states can with even angular momentum. 1 + be populated by one-photon absorption, but the Q1 g states only through absorption or emission of an even number of A. Doubly differential photoelectron-proton kinetic photons (e.g., one xuv photon plus one IR photon). energy spectra The calculated doubly differential photoelectron-proton III. RESULTS kinetic energy spectra obtained by using the APT-pump–IR- 1 + To study the dynamics of the Q1 u resonances, we probe scheme described in the previous section are shown ∗∗ → will focus on the dissociative ionization process H2 in Fig. 2 for different time delays. For comparison, the H + H+ + e−. The observables will be the electron kinetic figure also shows the results obtained with a SAP and no

063414-3 R. E. F. SILVA, P. RIVIERE,` AND F. MARTIN´ PHYSICAL REVIEW A 85, 063414 (2012)

IR pulse. This kind of representation, although difficult to achieve experimentally, contains very rich information on the photoionization dynamics, since it describes how energy is shared between electrons and nuclei (see, e.g., [29]). The dashed lines in the figure represent energy conservation for the different harmonic frequencies: for a given xuv-photon energyhω ¯ , where ω is the xuv-photon frequency, the excess energy Eω can be written as ω = − − = − E hω¯ (E∞ Eν0 ) hω¯ 18.1eV, (10) FIG. 3. (Color online) Photoelectron spectrum for the full calcu- where Eν0 is the energy of the ground rovibrational state of H2 1 + =− lation (left) and without Q1 resonances (right). Results without (Eν0 31.7 eV) and E∞ is the energy needed to reach the u ionization threshold (E∞ =−13.6 eV). For example, for the IR field (only APT) are shown at the left of the white vertical harmonic H21 (¯hω = 33.6 eV), the excess energy is EH 21 = line. The expected final energy upon absorption of a photon from 15.5 eV. This harmonic corresponds to the uppermost dashed each harmonic order is shown with horizontal lines. The square of line in the figures: all possible energy sharings between the the vector potential is shown in the upper part of the figures. All 6 electron and the proton upon absorption of a H21 photon lay probabilities are multiplied by 10 . on the straight line that goes from EKE = EH 21 and zero PKE to zero EKE and PKE = EH 21/2. process can be easily followed by analyzing the variation of In the result shown in Fig. 2(a) for a single attosecond the 2D spectrum with time delay [see the movie given in pulse, one can see two well defined regions. The first one, the Supplemental Material [30], from which the snapshots in which probabilities are largest, corresponds to very slow shown in Figs. 2(b)–2(d) have been extracted]. Indeed, as protons, i.e., to electrons that take almost all the excess energy. mentioned above, DESs have repulsive energy curves (see This region reflects direct ionization through the 1sσg channel. Fig. 1). Therefore, when one of these states is populated by The second region lies around the energy conservation lines, an xuv photon, the molecule starts to increase its internuclear drawn in this case only for comparison, and corresponds to distance, and it can either lead to dissociation into two neutral protons of intermediate energy. This region entirely reflects atoms or decay into the ionization continuum. The longer the autoionization from the DESs that have been populated by molecule has evolved in the dissociating curve, the higher will the xuv photon. At the highest proton energies, part of the be the kinetic energy gained by the nuclei. Consequently, the observed signal is due to direct photoionization through the higher the proton energy in a given energy-conservation line, 2pσu channel. This picture becomes much more complicated the larger the autoionization lifetime of the DES. when we consider the APT + IR scheme, Figs. 2(b)–2(d), Although these 2D PKE-EKE spectra are very useful which we discuss next. When the IR pulse comes before the to understand the photoionization process, it is much more APT, [Fig. 2(b), delay t =−T ], the 2D spectrum exclusively feasible experimentally to obtain integrated photoelectron and reflects the effect of the APT. This is because the IR pulse is proton kinetic energy spectra. We show results for both in the not intense enough to induce a significant amount of ionization next two subsections. or electronic excitation in H2 through multiphoton absorption. Thus, for all practical purposes, when the xuv APT arrives, B. Photoelectron spectra the H2 molecule is still in its ground state. After absorption of the xuv photons, a substantial part of the excess energy The photoelectron spectrum as a function of time delay goes again to the electrons and leads to the pronounced is shown in Fig. 3 both for the full calculation (left) and the 1 + signal at low proton energies. Nevertheless, this signal exhibits calculation without the Q1 u resonances (right). The results clear structures that follow the energy conservation lines that of a calculation in which only the APT was used (i.e., no represent the expected final energies upon ionization by the IR pulse) is shown at the left of the vertical dashed line in harmonics from EH 21 (uppermost line) down to EH 13. Along the left panels. The square of the vector potential is shown these energy conservation lines and for intermediate proton at the top of the figures. The horizontal lines on the right of energies, the ionization probability is also high, which is the photoelectron spectra indicate, for the different harmonic entirely due to autoionization of the DESs that have been energies, the energy available for the electron when the system populated by the xuv photon. Besides, at the highest proton is in the ν = 2 vibrational level of the 1sσg state of the + energies and very small EKE, there is the signature of direct H2 ion (with ν = 3, they have the highest population for photoionization into the 2pσu continuum. So, the 2D spectrum all delays). Since higher vibrational levels are also populated of Fig. 2(b) reflects exactly the same features as those of (corresponding to lower kinetic electron energies), these lines Fig. 2(a) for the SAP, but only along the energy conservation serve as an upper bound for the electron energy corresponding lines that correspond to the different harmonic energies. to photon absorption from each of the harmonics. 1 + When the APT and IR fields overlap, the APT drives As expected, the EKE spectra with and without the Q1 u first the system into the continuum whence absorption or resonances are almost identical because the dominant process emission of few IR photons is now possible. The latter is nondissociative direct ionization. The bands corresponding processes lead to final energies right in the middle between the to the different harmonics are clearly visible. When there is harmonic bands. The resulting sidebands are clearly visible for no IR, only the bands corresponding to the odd harmonics are t = 0 (c) and T/2 (d). The dynamics of the autoionization present. For negative delays (IR pulse before the xuv APT),

063414-4 AUTOIONIZING DECAY OF H2 DOUBLY EXCITED ... PHYSICAL REVIEW A 85, 063414 (2012)

FIG. 5. (Color online) Proton kinetic energy spectrum for the 1 + full calculation (left) and without Q1 u resonances (right). Results without IR ﬁeld (only APT) are shown at the left of the white vertical line. The square of the vector potential is shown in the upper part of the ﬁgures. All probabilities are multiplied by 106.

different from those already found in atoms [33,34]. This is simply because the EKE spectra are entirely dominated FIG. 4. (Color online) Same as in Fig. 3, but split into total by direct ionization without a significant dissociation of the symmetry u (top figures) and g (bottom figures). remaining molecular ion. the EKE is almost identical to that without the IR because, C. Proton kinetic energy spectra as mentioned above, the IR intensity is not high enough to The PKE spectra resulting from both the full calculation induce photoionization or significant electronic excitation, so 1 + (left) and that without the Q1 u resonances (right) are that when the APT arrives the H2 molecule is mostly in its ground state. When both fields overlap ( t ∼ 0), the molecule shown in Fig. 5 as a function of time delay. Both figures is first ionized by an xuv photon and then it can either absorb show an intense band of low proton energy (< 1eV), or emit IR photons, leading to the formation of sidebands which corresponds to direct dissociative ionization through the 1sσg channel. There is also a faint contribution at proton located at even harmonics of the generating frequency. For ∼ positive delays, the xuv comes first and the resulting molecular energies E 7.75 eV,which corresponds to direct dissociative ion evolves in time until the IR photon is absorbed. The ionization through the 2pσu channel. This channel is open resulting bands and sidebands vary with the time delay with a within the Franck-Condon region for the harmonic H21. period T/2. This periodicity has already been observed [23] For t > 3.5 fs, the low energy band widens, which is the and explained [31,32] in atomic systems when similar setups signature of bond softening: the nuclear wave packet launched by the xuv field in the 1sσg state evolves to higher internuclear have been used. It has also been recently observed in the H2 molecule [25]. When t FWHM there is no more overlap distances, where the 2pσu state is much closer in energy and between the pulses: the molecule absorbs an xuv photon and it is therefore accessible after absorption of a few IR photons. the ejected electron is not affected by the IR field, so the EKE This effect has been previously observed in H2 by using a is again maximum at odd harmonics. SAP-pump–IR-probe scheme [20]. The signal associated with In order to understand the dynamics of this process we autoionization from the DESs is clear in the results of the can split the EKE spectra into contributions from different symmetries: u and g. This is shown in Fig. 4:thetoptwo figures show the dissociative ionization probabilities for the u symmetry, which can be directly accessed by absorption of a single xuv photon (i.e., the odd harmonic bands), and the bottom two figures those for the g symmetry, which need the emmission or absorption of at least one IR photon (i.e., the sidebands). The latter are only significantly populated when both pulses overlap. Note that for each symmetry there is a clear pattern with T/2 periodicity. When the transition probability to the sidebands (bottom) is maximized, there is a depletion in the odd harmonic bands (top). This occurs when the combined absorption of an xuv and an IR photon is efficient, i.e., when the attosecond pulses of the APT lie around the maxima of the IR field. The combination of these two clear patterns produces the complicated spectrum in Fig. 3. Apart from the expected broadening of the different bands and sidebands due to the sharing of the excess energies between the ejected electron and the nuclear vibrational states of the FIG. 6. (Color online) Same as in Fig. 5, but split into total + final H2 molecular ion, these results are not substantially symmetry u (top figures) and g (bottom figures).

063414-5 R. E. F. SILVA, P. RIVIERE,` AND F. MARTIN´ PHYSICAL REVIEW A 85, 063414 (2012)

electronic degrees of freedom of the molecule. To simplify the analysis and the calculations, the case of H2 molecules oriented parallel to the polarization direction has been considered. We did three series of calculations: with all Q1 resonances, 1 + 1 + without the Q1 u states, and without the Q1 g states. The last set of calculations leads to results that are almost identical to those from the full calculation, showing that by using this pump-probe scheme, the population of the optically forbidden + g resonances is neglible. Our spectral method also allows us + + to analyze separately the contribution of the g and u ﬁnal FIG. 7. (Color online) Same as in Fig. 5, but including only the symmetries. 1sσg continuum. We have shown results for doubly differential photoelectron-proton kinetic energy spectra (or 2D coinci- full calculation (Fig. 5, left): it appears at intermediate proton dence spectra, for simplicity) in the case of a SAP, as well energies (1–6 eV) with T/2 periodicity and disappears when + 1 + as for the APT IR scheme at different time delays. The the Q1 u states are removed from the calculations (Fig. 5, autoionization dynamics can be easily followed by looking right). at the variation of the 2D spectrum with time delay. We The u and g contributions to the above spectra are have also shown results for the integrated electron kinetic shown in Fig. 6. In the full calculation for the u symmetry, 1 + energy (EKE) and proton kinetic energy (PKE) spectra. In the one can clearly see autoionization from the Q1 u DESs to EKE spectrum, we observe a complicated pattern that can be the continuum of the same symmetry at intermediate proton + + 1 + easily understood by analyzing the g and u contributions energies, while this process is totally absent when the Q1 u separately. Autoionization leaves almost no trace in the EKE states are removed from the calculations. In the g symmetry, spectrum, but it plays a dominant role in PKE spectrum since one can also observe a faint contribution from these resonances it is responsible for almost all the protons ejected in the 1–6 eV at intermediate proton energies resulting from the radiative 1 + energy region. decay of the Q1 u DESs to the 1sσg(g) continuum upon In summary, APT-pump–IR-probe schemes are a useful absorption or emission of an IR photon; see Fig. 1. tool to track autoionization dynamics in molecules. An The signature of autoionization can be seen even more alternative setup, such as (APT + IR)-pump–IR-probe, would clearly if we remove the contribution of the 2pσu continuum be necessary to probe doubly excited states of 1+ symmetry. from the PKE. The result is shown in Fig. 7, that should be g compared with Fig. 5: when the 2pσu contribution is removed, both the band at high energies and the bond softening at long ACKNOWLEDGMENTS delays disappear, and only the resonances remain. This work was accomplished with an allocation of computer time from Mare Nostrum BSC and CCC-UAM, and was IV. CONCLUSIONS partially supported by the MICINN Projects No. FIS2010- In this work we have studied the autoionization dynamics 15127, No. ACI2008-0777, and No. CSD 2007-00010, the of the ﬁrst series of doubly excited states of the H2 molecule, ERA-Chemistry Project No. PIM2010EEC-00751, the Euro- 1 + the Q1 u series, by using an APT-pump–IR-probe scheme pean grants MC-ITN CORINF and MC-RG ATTOTREND, in which the xuv photons of the APT are used to populate the the European COST Action CM0702, and the Advanced doubly excited states and the dynamics is tracked by changing Grant of the European Research Council XCHEM 290853. the delay between the APT pump and the IR probe. For this, we R.E.F.S. acknowledges a Ph.D. contract from ITN CORINF. have solved the time-dependent Schrodinger¨ equation by using P.R. acknowledges a Juan de la Cierva contract grant from a close-coupling method that accounts for all vibrational and MICINN.

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