We investigate the possibility of using molecular alignment for controlling the relative probability of individual reaction pathways in polyatomic molecules initiated by electronic processes on the few-femtosecond time scale. Using acetylene as an example, it is shown that aligning the molecular axis with respect to the polarization direction of the ionizing laser pulse does not only allow us to enhance or suppress the overall fragmentation yield of a certain fragmentation channel but, more importantly, to determine the relative probability of individual reaction pathways starting from the same parent molecular ion. We show that the achieved control over dissociation or isomerization pathways along specific nuclear degrees of freedom is based on a controlled population of associated excited dissociative electronic states in the molecular ion due to relatively enhanced ionization contributions from inner valence orbitals.
We visualize and control molecular dynamics taking place on intermediately populated states during different sequential double ionization pathways of CO2 using a sequence of two delayed laser pulses which exhibit different peak intensities. Measured yields of CO2 (2+) and of fragment pairs CO(+)/O(+) as a function of delay between the two pulses are weakly modulated by various vibronic dynamics taking place in CO2 (+). By Fourier analysis of the modulations we identify the dynamics and show that they can be assigned to merely two double ionization pathways. We demonstrate that by reversing the sequence of the two pulses it becomes possible to control the pathway which is taken across CO2 (+) towards the final state in CO2 (2+). A comparison between the yields of CO2 (2+) and CO(+)/O(+) reveals that the modulating vibronic dynamics oscillate out-of-phase with each other, thus opening up opportunities for strong-field fragmentation control on extended time scales.
We report on the measurement of electron emission after the interaction of strong laser pulses with atoms and molecules. These electrons originate from high-lying Rydberg states with quantum numbers up to n 120 formed by frustrated field ionization. Simulations show that both tunneling ionization by a weak dc field and photoionization by the black-body radiation contribute to delayed electron emission on the nano-to microsecond scale. We measured ionization rates from these Rydberg states by coincidence spectroscopy. Further, the dependence of the Rydberg-state production on the ellipticity of the driving laser field proves that such high-lying Rydberg states are populated through electron recapture. The present experiment provides detailed quantitative information on Rydberg production by frustrated field ionization.PACS numbers: 32.80. Rm, 32.80.Fb, 42.50.Hz Ionization of atoms and molecules by strong laser fields is the starting point for a multitude of interesting phenomena, e.g., high harmonic generation or molecular fragmentation [1]. For sufficiently strong laser fields corresponding to intensities of the order of I ≈ 10 14 W/cm 2 , atoms and molecules are ionized via tunneling ionization, i.e., an electron passes through the potential barrier of the combined Coulomb and laser fields. After tunneling, electrons are steered by the laser field and most of them will eventually escape the Coulomb field of the remaining ion core. However, a fraction of them are recaptured into highly excited states by the ionic Coulomb field. This process frequently referred to frustrated field ionization (FFI) [2,3] leads to the formation of high-lying Rydberg states with binding energies extending from a fraction of an eV to values of µeV near threshold.Very high-lying Rydberg states with principal quantum numbers n ≈ 100 are quantum objects of macroscopic size allowing for studies of the border between the quantum and the classical worlds [4]. Formation and destruction of such mesoscopic objects can be described by semiclassical and classical methods [5]. Recent experiments on high harmonic generation and electron wave packet interferometry indicate the important contribution of such excited states to different processes [6][7][8][9][10] including ionization and molecular dissociation processes [11][12][13][14]. However, detailed and quantitative information on the FFI following the interaction of femtosecond laser pulses with atoms and molecules appears to be scarce.To explore the production process and the properties of high-lying Rydberg states formed in the strong field interaction with atoms and molecules, direct observation of such states is required. Traditionally, zero kinetic energy photoelectron spectroscopy is applied to study weakly bound states in atoms and molecules [15]. In case of strong field interaction, however, the ionization signal from Rydberg states is completely overshadowed by the dominant laser field-induced ionization signal from the target and the residual gas in the interaction chamber. There...
Oxygen (O2 ) is one of the most important elements required to sustain life. The concentration of O2 on Earth has been accumulated over millions of years and has a direct connection with that of CO2 . Further, CO2 plays an important role in many other planetary atmospheres. Therefore, molecular reactions involving CO2 are critical for studying the atmospheres of such planets. Existing studies on the dissociation of CO2 are exclusively focused on the C-O bond breakage. Here we report first experiments on the direct observation of molecular Oxygen formation from CO2 in strong laser fields with a reaction microscope. Our accompanying simulations suggest that CO2 molecules may undergo bending motion during and after strong-field ionization which supports the molecular Oxygen formation process. The observation of the molecular Oxygen formation from CO2 may trigger further experimental and theoretical studies on such processes with laser pulses, and provide hints in studies of the O2 and O + 2 abundance in CO2 -dominated planetary atmospheres.PACS numbers: 33.80. Gj, 42.50.Hz, O 2 production is one of the most important processes for the biosphere of the Earth.Oxygen molecules are mainly generated via the photosynthesis by green plants and algae from carbon dioxide and water:. CO 2 is not only important for the atmosphere on Earth, it is also the dominant compound of the atmosphere on other planets, such as Mars and Venus. One of the most crucial tasks for the quest to establish a human settlement on Mars is the production of O 2 [2]. Because more than 95% of the atmosphere on Mars is CO 2 , it will be extremely helpful if O 2 can be produced directly from CO 2 . In the past, it was observed that dissociation of CO 2 via absorption of photons leads to carbon monoxide (CO) and oxygen atoms (O) [3]. However, theoretical simulations suggested the possibility of generating O 2 through the dissociation of a CO 2 molecule [4]. A recent experiment showed the evidence of O 2 formation from CO 2 molecules after UV excitation through the detection of C + [5]. So far, O 2 formation from CO 2 has not been directly observed.In the past decade, intense ultrashort laser pulses have been successfully applied to trigger and control molecular reactions such as dissociation and isomerization [6][7][8][9][10][11][12][13][14][15]. When a molecule interacts with a strong laser field, electrons from outer molecular orbitals can be excited or removed through tunneling or over-the-barrier ionization which may prepare the molecule in an excited state or a state with a certain charge. As a consequence, the excited or ionized molecule may undergo severe geometrical reconfiguration and may also break into several fragments or form new chemical bonds. Because of the importance of CO 2 in many research disciplines, strongfield induced reactions of CO 2 have been experimentally studied with ultrashort lasers by several research groups.However, these studies mainly focused on the topic of ionization and dissociation [16]. In this paper, for the first...
The alignment-dependent ionization of acetylene and ethylene in short laser pulses is investigated in the framework of the time-dependent density-functional theory coupled with Ehrenfest dynamics. The molecular alignment is found to have a substantial effect on the total ionization. Bond stretching is shown to cause an increase of the ionization efficiency, i.e., enhanced ionization, in qualitative agreement with previous theoretical investigations. It is also demonstrated that the enhanced ionization mechanism greatly enhances the ionization from the inner valence orbitals, and the ionization of the inner orbitals is primarily due to their extended weakly bound density tails.
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