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Experimental observation of the elusive double-peak structure in R-dependent strong-field ionization rate of H2(+).

Xu H, He F, Kielpinski D, Sang RT, Litvinyuk IV - Sci Rep (2015)

Bottom Line: In the experiment, an expanding H2(+) ion produced by an intense pump pulse is probed by a much weaker probe pulse.The predicted double-peak structure is clearly seen in delay-dependent kinetic energy spectra of protons when pump and probe pulses are polarized parallel to each other.No structure is seen when the probe is polarized perpendicular to the pump.

View Article: PubMed Central - PubMed

Affiliation: Centre for Quantum Dynamics and Australian Attosecond Science Facility, Griffith University, Nathan, QLD 4111, Australia.

ABSTRACT
When a diatomic molecule is ionized by an intense laser field, the ionization rate depends very strongly on the inter-nuclear separation. That dependence exhibits a pronounced maximum at the inter-nuclear separation known as the "critical distance". This phenomenon was first demonstrated theoretically in H2(+) and became known as "charge-resonance enhanced ionization" (CREI, in reference to a proposed physical mechanism) or simply "enhanced ionization"(EI). All theoretical models of this phenomenon predict a double-peak structure in the R-dependent ionization rate of H2(+). However, such double-peak structure has never been observed experimentally. It was even suggested that it is impossible to observe due to fast motion of the nuclear wavepackets. Here we report a few-cycle pump-probe experiment which clearly resolves that elusive double-peak structure. In the experiment, an expanding H2(+) ion produced by an intense pump pulse is probed by a much weaker probe pulse. The predicted double-peak structure is clearly seen in delay-dependent kinetic energy spectra of protons when pump and probe pulses are polarized parallel to each other. No structure is seen when the probe is polarized perpendicular to the pump.

No MeSH data available.


Related in: MedlinePlus

(a) Measured delay-dependent KER spectrum of the EI channel (same as in Fig. 2(a)) compared with the simulated delay-dependent KER spectrum (b). (c) The corresponding simulated delay-dependent energy-integrated ionization rate (see Methods for details). A periodic modulation of proton yield at a period of the driving laser (~2.5 fs) is also visible in the spectra. As confirmed by our numerical simulation, such modulation comes from the interference between the tail of the pump pulse and the probe pulse.
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f3: (a) Measured delay-dependent KER spectrum of the EI channel (same as in Fig. 2(a)) compared with the simulated delay-dependent KER spectrum (b). (c) The corresponding simulated delay-dependent energy-integrated ionization rate (see Methods for details). A periodic modulation of proton yield at a period of the driving laser (~2.5 fs) is also visible in the spectra. As confirmed by our numerical simulation, such modulation comes from the interference between the tail of the pump pulse and the probe pulse.

Mentions: This time-dependent double-peak structure in the yield, in conjunction with the time-dependent KER identifying the channel, presents sufficient proof of the double-peak structure in R-dependent ionization probability. In order to relate the time delays to the inter-nuclear separations and to confirm our assignment we also performed a numerical simulation (see Methods for details). The results of that simulation are presented in Fig. 3. The simulated delay-dependent KER spectra for enhanced ionization channel have the same structure as observed in the experiment (Fig. 3(a)): a strong branch with delay dependent KER and a weak branch with delay-independent KER are present in the simulation as well. However, our model fails to quantitatively reproduce relative heights of the two peaks, most likely due to its reduced dimensionality with the electron motion constrained to the laser polarization direction. There are some theoretical indications that off-axis electron trajectories contribute significantly to the second peak11, implying that a quantitative theoretical description will require a full-dimensional treatment of electrons. The periodic modulation of the yield with a period equal to the laser cycle (2.7 fs) seen in the experiment is due to interference of the probe pulse with the extended pedestal of the pump pulse. Including such pedestal in the modelled pump pulse (see Methods) faithfully reproduces this modulation. Since no enhanced ionization was observed due to the probe pulse alone, we can conclude that the actual pedestal of the pump pulse at relevant delays from its peak is much weaker than the probe pulse and aside from the laser-cycle modulation the pedestal does not affect the underlying physics and our conclusions.


Experimental observation of the elusive double-peak structure in R-dependent strong-field ionization rate of H2(+).

Xu H, He F, Kielpinski D, Sang RT, Litvinyuk IV - Sci Rep (2015)

(a) Measured delay-dependent KER spectrum of the EI channel (same as in Fig. 2(a)) compared with the simulated delay-dependent KER spectrum (b). (c) The corresponding simulated delay-dependent energy-integrated ionization rate (see Methods for details). A periodic modulation of proton yield at a period of the driving laser (~2.5 fs) is also visible in the spectra. As confirmed by our numerical simulation, such modulation comes from the interference between the tail of the pump pulse and the probe pulse.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4551962&req=5

f3: (a) Measured delay-dependent KER spectrum of the EI channel (same as in Fig. 2(a)) compared with the simulated delay-dependent KER spectrum (b). (c) The corresponding simulated delay-dependent energy-integrated ionization rate (see Methods for details). A periodic modulation of proton yield at a period of the driving laser (~2.5 fs) is also visible in the spectra. As confirmed by our numerical simulation, such modulation comes from the interference between the tail of the pump pulse and the probe pulse.
Mentions: This time-dependent double-peak structure in the yield, in conjunction with the time-dependent KER identifying the channel, presents sufficient proof of the double-peak structure in R-dependent ionization probability. In order to relate the time delays to the inter-nuclear separations and to confirm our assignment we also performed a numerical simulation (see Methods for details). The results of that simulation are presented in Fig. 3. The simulated delay-dependent KER spectra for enhanced ionization channel have the same structure as observed in the experiment (Fig. 3(a)): a strong branch with delay dependent KER and a weak branch with delay-independent KER are present in the simulation as well. However, our model fails to quantitatively reproduce relative heights of the two peaks, most likely due to its reduced dimensionality with the electron motion constrained to the laser polarization direction. There are some theoretical indications that off-axis electron trajectories contribute significantly to the second peak11, implying that a quantitative theoretical description will require a full-dimensional treatment of electrons. The periodic modulation of the yield with a period equal to the laser cycle (2.7 fs) seen in the experiment is due to interference of the probe pulse with the extended pedestal of the pump pulse. Including such pedestal in the modelled pump pulse (see Methods) faithfully reproduces this modulation. Since no enhanced ionization was observed due to the probe pulse alone, we can conclude that the actual pedestal of the pump pulse at relevant delays from its peak is much weaker than the probe pulse and aside from the laser-cycle modulation the pedestal does not affect the underlying physics and our conclusions.

Bottom Line: In the experiment, an expanding H2(+) ion produced by an intense pump pulse is probed by a much weaker probe pulse.The predicted double-peak structure is clearly seen in delay-dependent kinetic energy spectra of protons when pump and probe pulses are polarized parallel to each other.No structure is seen when the probe is polarized perpendicular to the pump.

View Article: PubMed Central - PubMed

Affiliation: Centre for Quantum Dynamics and Australian Attosecond Science Facility, Griffith University, Nathan, QLD 4111, Australia.

ABSTRACT
When a diatomic molecule is ionized by an intense laser field, the ionization rate depends very strongly on the inter-nuclear separation. That dependence exhibits a pronounced maximum at the inter-nuclear separation known as the "critical distance". This phenomenon was first demonstrated theoretically in H2(+) and became known as "charge-resonance enhanced ionization" (CREI, in reference to a proposed physical mechanism) or simply "enhanced ionization"(EI). All theoretical models of this phenomenon predict a double-peak structure in the R-dependent ionization rate of H2(+). However, such double-peak structure has never been observed experimentally. It was even suggested that it is impossible to observe due to fast motion of the nuclear wavepackets. Here we report a few-cycle pump-probe experiment which clearly resolves that elusive double-peak structure. In the experiment, an expanding H2(+) ion produced by an intense pump pulse is probed by a much weaker probe pulse. The predicted double-peak structure is clearly seen in delay-dependent kinetic energy spectra of protons when pump and probe pulses are polarized parallel to each other. No structure is seen when the probe is polarized perpendicular to the pump.

No MeSH data available.


Related in: MedlinePlus