Probing Molecular Dynamics at Attosecond Resolution with Femtosecond Laser Pulses (original) (raw)
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Sub-laser-cycle electron pulses for probing molecular dynamics
Experience shows that the ability to make measurements in any new time regime opens new areas of science. Currently, experimental probes for the attosecond time regime (10 218 –10 215 s) are being established. The leading approach is the generation of attosecond optical pulses by ionizing atoms with intense laser pulses. This nonlinear process leads to the production of high harmonics during collisions between electrons and the ionized atoms. The underlying mechanism implies control of energetic electrons with attosecond precision. We propose that the electrons themselves can be exploited for ultrafast measurements. We use a 'molecular clock', based on a vibrational wave packet in H 2 1 to show that distinct bunches of electrons appear during electron–ion collisions with high current densities, and durations of about 1 femtosecond (10 215 s). Furthermore, we use the molecular clock to study the dynamics of non-sequential double ionization. A substantial effort is under way to develop single attosecond optical pulses 1–3 , or trains of attosecond pulses 4–6 , using the physical processes occuring in high-harmonic generation 7. High harmonics are produced during the electron–ion collisions induced by strong-field laser ionization, usually referred to as 'recollision'. Within one optical period an electron is removed from the atom, is driven back when the laser field reverses its direction, and collides with the parent ion. The duration of the electron–ion recollision largely determines the duration of the attosecond photon pulse. Here we study the recollision electron wave packet, measuring both the probability of recollision and its time structure. Although only one electron is involved in the electron–ion recollision, we adopt the language of electron beams to indicate the potential applications of recollision electrons. These applications are the topic of the final section of this Article. We characterize the unusually large current density and its time structure as seen by the ion following ionization. To do this, we use H 2 molecules in a low-density gas as a molecular clock. As ionization simultaneously forms two wave packets—one a vibrational wave packet moving on the H 2 þ (X 2 S g þ) surface; the other, the electron wave packet that we wish to study—ionization starts the vibrational clock in H þ 2 : We choose H þ 2 as the molecular clock because of the speed of its vibrational wave packet, and because all excited states of H þ 2 directly dissociate. By choosing the molecular axis perpendicular to the laser electric field, we decouple the X 2 S þ g and A 2 S þ u surfaces in H þ 2 ; ensuring that the clock remains accurate in the presence of the field. To read the clock, we observe the kinetic energy of the protons produced by inelastic scattering when the electron recollides with the parent ion. The kinetic energy distribution measures the position of the vibrational wave packet at the time of recollision, and therefore the recollision time. In our experiment the time resolution is ,1 fs. Next, we apply the molecular clock to follow the subcycle correlated electron dynamics. Non-sequential double ionization (two-electron ionization that cannot be described by two sequential single-electron ionization processes) is a common occurrence during strong-field ionization of atoms or molecules containing two or more electrons 7–13. We distinguish the double ionization due to recollision from instantaneous double ionization by using the molecular clock, and find that electron recollision dominates others by at least two orders of magnitude. We confirm that the most important route to non-sequential ionization is through the production of excited states by recollision that can later ionize in the strong laser field. Finally, we compare the double-ionization yield due to recollision in H 2 and helium 8,11,13. We find that double ionization (excitation) is about ten times more probable in hydrogen molecules than in helium 11,13. Selecting the fragmentation channel We now proceed to fully characterize the current density using H 2 double-ionization (excitation) for all aspects of the measurement. (For convenience, we will use 'double-ionization' when referring to either the non-sequential emission of two electrons, or the emission of one and the correlated excitation of the other.) First, we identify collision-induced excitation or double ioniz-ation through the previously observed 14,15 high kinetic energy of the fragment protons that are produced. We show that recollision is responsible for these energetic protons by comparing the kinetic-energy spectrum measured with linear and elliptically polarized light 16. Second, the ellipticity dependence of the proton yield measures the initial velocity spread of the electron wave packet. With this input, we calculate the current density seen by the newly ionized ion. Third, we confirm the predicted temporal structure by comparing the calculated and measured kinetic-energy spectrum. Finally, we confirm the magnitude of the current density by
Physical Review A, 2003
We studied the recollision dynamics between the electrons and D + 2 ions following the tunneling ionization of D2 molecules in an intense short pulse laser field. The returning electron collisionally excites the D + 2 ion to excited electronic states from there D + 2 can dissociate or be further ionized by the laser field, resulting in D + + D or D + + D + , respectively. We modeled the fragmentation dynamics and calculated the resulting kinetic energy spectrum of D + to compare with recent experiments. Since the recollision time is locked to the tunneling ionization time which occurs only within fraction of an optical cycle, the peaks in the D + kinetic energy spectra provides a measure of the time when the recollision occurs. This collision dynamics forms the basis of the molecular clock where the clock can be read with attosecond precision, as first proposed by Corkum and coworkers. By analyzing each of the elementary processes leading to the fragmentation quantitatively, we identified how the molecular clock is to be read from the measured kinetic energy spectra of D + and what laser parameters be used in order to measure the clock more accurately.
Attosecond xuv pulses for complete mapping of the time-dependent wave packets of D_{2}^{+}
Physical Review A, 2006
We have shown that the whole time-dependent vibrational wave packet of D 2 + ions can be reconstructed from the kinetic energy release of the D + ion pairs when it is probed with an attosecond xuv pulse. Such a full interrogation of the wave packet will pave the way for controlling the generation of tailor-designed wave packets for favorable chemical reaction paths, as well as for probing the time evolution of their interaction with the medium.
Femtosecond time-resolved spectroscopy of elementary molecular dynamics
Naturwissenschaften, 2002
Femtosecond time-resolved coherent anti-Stokes Raman spectroscopy (CARS) is applied in order to prepare and monitor laser-induced vibrational coherences (wave packets) of different samples mainly in its electronic ground state but also in excited states. The time evolution of these wave packets gives information on the dynamics of molecular vibrations. In a first example the femtosecond (fs) CARS transients of iodine are investigated. By changing the relative delay between the applied laser pulses of this non-degenerated four-wave mixing technique, both the wavepacket motion on the electronically excited and the ground states can be detected as oscillations in the coherent anti-Stokes signal. Second we report on selective excitation of the vibrational modes in the electronic ground state of polymers of diacetylene by means of a femtosecond time-resolved CARS scheme. This selectivity is achieved by varying the phase shape (chirp) and the relative delay between the exciting laser pulses.
Probing the Dynamics of a Molecular Ion with Laser Pulses
Laser Physics
The dynamics of a molecular ion driven by two laser pulses separated by a time lag is studied beyond the Born-Oppenheimer approximation. The first, short, pulse prepares the molecule in some quantum state, which is probed by the second pulse. Under suitable conditions, the molecule emits a spectrum of redshifted high-order harmonics. The value of the redshift is proportional to the harmonic order and can be used as a measure of the speed of the atoms of the molecule.
The dynamical behaviour of H2 and D2 in a strong, femtosecond laser field
Detailed measurements of H 2 and D 2 dissociation fragment kinetic energy dependences on laser intensity, using 150 fs, 800 nm pulses, are presented. The yields for both molecular and atomic ions are also given. The observed three-peak kinetic energy spectrum carries within it the signature of the different stages of the interaction. The two lower energy peaks are a product of bond softening (and above threshold) dissociation of the molecular ion from Franck-Condon populated vibrational levels. The third higher-energy peak results from enhanced ionization of the dissociating molecular ions. No light-induced vibrational trapping need be invoked to interpret the higher-energy fragments.
Probing molecular dynamics with attosecond resolution using correlated wave packet pairs
Spectroscopic measurements with increasingly higher time resolution are generally thought to require increasingly shorter laser pulses, as illustrated by the recent monitoring of the decay of core-excited krypton1 using attosecond photon pulses2,3. However, an alternative approach to probing ultrafast dynamic processes might be provided by entanglement, which has improved the precision4,5 of quantum optical measurements. Here we use this approach to observe the motion of a D2 1 vibrational wave packet formed during the multiphoton ionization of D2 over several femtoseconds with a precision of about 200 attoseconds and 0.05 a°ngstro¨ms, by exploiting the correlation between the electronic and nuclear wave packets formed during the ionization event. An intense infrared laser field drives the electron wave packet, and electron recollision6–11 probes the nuclear motion. Our results show that laser pulse duration need not limit the time resolution of a spectroscopic measurement, provided the process studied involves the formation of correlated wave packets, one of which can be controlled; spatial resolution is likewise not limited to the focal spot size or laser wavelength.
Attosecond Time-Resolved Electron Dynamics in the Hydrogen Molecule
IEEE Journal of Selected Topics in Quantum Electronics, 2012
Recent advances in the generation and characterization of extreme-ultraviolet pulses, generated either by intense femtosecond lasers or by free electron lasers, are pushing the frontier of time-resolved investigations down to the attosecond domain, the relevant timescale for electron motion. The quantum nature of the intertwined electronic and nuclear motion requires theoretical models going beyond the Born-Oppenheimer approximation and taking into account electron correlation, representing a challenge for the computational power available nowadays. Understanding how the electron dynamics inside molecules can influence chemical reactions presents important implications in several fields and allows for the development of new technologies. In this paper, we report on experimental and theoretical results of an investigation in H2/D2, where for the first time control of molecular dynamics with attosecond resolution was achieved. The data represent the first evidence of the control of the electron motion in a molecule undergoing a chemical reaction on the subfemtosecond scale.
Doctor of PhilosophyDepartment of PhysicsArtem RudenkoPhotoelectron spectroscopy employing X-ray and extreme ultraviolet (XUV) radiation is one of the most important experimental methods to study the electronic structure of atoms, molecules, and solids. Recent developments of XUV and X-ray sources with ultrashort pulse durations, like free-electron lasers (FELs) and high-order harmonics of infrared lasers, enabled combining this approach with a concept of a time-resolved measurement, where a pair of synchronized short light pulses is used to initiate and observe a physical or chemical process of interest. Among other advances, such combination turned out to be particularly useful for atomic physics and gas-phase femtochemistry, where femtosecond or even sub-femtosecond short-wavelength radiation can be used to trigger the dynamics in high-lying states previously inaccessible for time-resolved measurements and offers a variety of novel schemes to probe light-induced electronic and nu...