Excitation processes during strong- field ionization and dissociatation of molecules Grad students: Li Fang, Brad Moser Funding : NSF-AMO November 29, 2006 ISUILS5 Lijiang, China George N. Gibson University of Connecticut Department of Physics

Motivation Excitation of molecules by strong laser fields is not well-studied. Excitation of molecules by strong laser fields is not well-studied. Excitation can have positive benefits, such as producing inversions in the VUV and providing spectroscopy of highly excited states of molecules. Excited states of H 2 + have never been studied before! Excitation can have positive benefits, such as producing inversions in the VUV and providing spectroscopy of highly excited states of molecules. Excited states of H 2 + have never been studied before! Can be detrimental to certain applications, such as quantum tomography of molecular orbitals. Can be detrimental to certain applications, such as quantum tomography of molecular orbitals.

Outline Ionization of I 2 to the A 2  state of I 2 +. Ionization of I 2 to the A 2  state of I 2 +. Excitation of H 2 + from 2p  u to 2s  g via a 7- photon resonance at 800 nm. Excitation of H 2 + from 2p  u to 2s  g via a 7- photon resonance at 800 nm. Tuning experiment showing resonant excitation from the 1s  g and 2p  u states to 2p  u. Tuning experiment showing resonant excitation from the 1s  g and 2p  u states to 2p  u. Theory of high-order multiphoton excitation. Theory of high-order multiphoton excitation. Conclusions. Conclusions.

Inner-orbital ionization In TOF, we measure I 2 → I 2 +. If the I 2 + does not dissociate, we have no idea what electronic state it is left in. In TOF, we measure I 2 → I 2 +. If the I 2 + does not dissociate, we have no idea what electronic state it is left in. This is a general problem with TOF spectroscopy, except for one exception: (I 2 2+ )* → I 2+ + I 0+, i.e. asymmetric dissociation. This is a general problem with TOF spectroscopy, except for one exception: (I 2 2+ )* → I 2+ + I 0+, i.e. asymmetric dissociation. Can look for fluorescence, but more on that later. Can look for fluorescence, but more on that later. In TOF, we have found the we can identify excited states by their vibration signature in pump-probe experiments. In TOF, we have found the we can identify excited states by their vibration signature in pump-probe experiments.

Laser System Ti:Sapphire 800 nm Oscillator Ti:Sapphire 800 nm Oscillator Regenerative or Multipass Amplifier Regenerative or Multipass Amplifier 750  J 1 KHz 750  J 1 KHz Transform Limited, 25 fs pulses Transform Limited, 25 fs pulses Can double to 400 nm Can double to 400 nm Have a pump-probe setup Have a pump-probe setup

Ion Time-of-Flight Spectrometer

I 2 potential energy curves

I 2+ pump-probe data

Simulation of A state

Conclusions from I 2 Can identify excited molecular states from vibrational signature. Can identify excited molecular states from vibrational signature. Can learn about the strong-field tunneling ionization process, especially details about the angular dependence. Can learn about the strong-field tunneling ionization process, especially details about the angular dependence. Could be a major problem for quantum tomography. Could be a major problem for quantum tomography.

VUV Detection Scheme Vertical Focal Volume Vertical Focal Volume ~ 45  m diameter ~ 45  m diameter ~ W/cm 2 ~ W/cm 2 Iridium Imaging Optic Iridium Imaging Optic 2 inch F/ inch F/1.33 R ~ 20% R ~ 20% Timed Correlated Photon Counting (TCPC) – 5 ns resolution Timed Correlated Photon Counting (TCPC) – 5 ns resolution

Pressure Dependence of L 

Fluorescence quenching

Quenching with circular polarization – rules out rescattering

We have a direct excitation of H 2 + which is not due to rescattering. We have a direct excitation of H 2 + which is not due to rescattering. The excitation efficiency is about 1%. The excitation efficiency is about 1%.

Wavelength tuning experiments Little work done on strong-field processes with a tunable laser. The assumption is that we are generally in the tunneling regime where the photon energy is not significant. Little work done on strong-field processes with a tunable laser. The assumption is that we are generally in the tunneling regime where the photon energy is not significant. However, we have shown that high-order resonant processes can be quite strong. If true, there should be a resonant signature. However, we have shown that high-order resonant processes can be quite strong. If true, there should be a resonant signature. Recently brought online an optical parametric amplifier – a TOPAS. Recently brought online an optical parametric amplifier – a TOPAS.

Resonances in H 2 + There is a problem with looking for resonances in H 2 + : ion is dissociating when it reaches the region of strong-field coupling. Curves are all changing, so resonances will be smeared out. There is a problem with looking for resonances in H 2 + : ion is dissociating when it reaches the region of strong-field coupling. Curves are all changing, so resonances will be smeared out. However, the 2p  u state is quite parallel to the ground states, so we might see a resonance. However, the 2p  u state is quite parallel to the ground states, so we might see a resonance.

Tuning data in H 2.

Conclusions from VUV data We see Lyman-  radiation following strong-field ionization of H 2. We see Lyman-  radiation following strong-field ionization of H 2. From the linear pressure dependence and the pump-probe quenching experiment, we know the excitation is a direct strong-field effect. From the linear pressure dependence and the pump-probe quenching experiment, we know the excitation is a direct strong-field effect. From the tuning experiment, we know the excitation is resonant. From the tuning experiment, we know the excitation is resonant. From this, we know that we have 4, 5, and 7 photon transitions in H 2 + driven by the laser field. From this, we know that we have 4, 5, and 7 photon transitions in H 2 + driven by the laser field.

What is so special about charged diatomic molecules? Ground state is a far off-resonant covalent state. Above this is a pair of strongly coupled ionic states. Only a weak coupling between them. If the ionic states are populated, there will probably be inversions.

3-Level Model System This system can be solved exactly for the n-photon Rabi frequency!

But, what is the physics? First, consider just the coupling between levels 2 and 3: This gives: and let

Expand in Bessel Functions: The eigenvalues are linear in the field, so their time average is zero, so there is no AC Stark shift. The Fourier components are at ±n , exactly.

“Floquet Ladder” The pair of degenerate states are strongly modulated by the laser field and create a complete Floquet ladder of states – with no ac Stark shift! The ground state couples to this through a 1-photon process which only produces a small Stark shift.

Key is a linear response to an external field – flux qubits?! 20-photon transitions seen! 20-photon transitions seen! Energy levels

5-photon transition with a pulsed laser

Adiabatic passage on a 10-photon transition

Conclusions We see Lyman-  radiation produced through the direction excitation of H 2 +. We see Lyman-  radiation produced through the direction excitation of H 2 +. We propose that the excitation comes from a resonant 7-photon transition driven by the degenerate strongly coupled states in H 2 +. We propose that the excitation comes from a resonant 7-photon transition driven by the degenerate strongly coupled states in H 2 +. This mechanism will also produce equal amounts of metastable hydrogen. This mechanism will also produce equal amounts of metastable hydrogen. Hydrogen is not the best candidate for excitation because of the high ionization rate of the excited state. However, it is easier to analyze. Hydrogen is not the best candidate for excitation because of the high ionization rate of the excited state. However, it is easier to analyze.

Ion TOF pump/probe experiment H 2 → H 2 + → H + + H(1s) → H + + H(2p). But, we have no hope of seeing H + + H(2p) on top of H + + H(1s). H 2 → H 2 + → H + + H(1s) → H + + H(2p). But, we have no hope of seeing H + + H(2p) on top of H + + H(1s). However, a weak pulse can ionize H(2p) but not H(1s). H + + H(2p) → H + + H + we can see through coincidence measurements. However, a weak pulse can ionize H(2p) but not H(1s). H + + H(2p) → H + + H + we can see through coincidence measurements. So, we are looking for a low energy H + + H + correlation. So, we are looking for a low energy H + + H + correlation.

Correlated H + ion spectrum

Intensity dependence of correlated signal

Conclusions from TOF measurement Can detect a low energy H + + H + correlation. Can detect a low energy H + + H + correlation. Energy is ~2.7 eV, exactly as expected from the proposed excitation pathway. Energy is ~2.7 eV, exactly as expected from the proposed excitation pathway. Population of H(2p) relative to H(1s) is ~1%. Population of H(2p) relative to H(1s) is ~1%. Ionization competes with excitation. Ionization competes with excitation.