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The Physics of Charge-Asymmetric Molecular States

The Physics of Charge-Asymmetric Molecular States. Why I never let go of my Ph.D. thesis research!. Rhodes Scholars Symposium University of Illinois, Chicago March 28, 2012. Supported by: National Science Foundation Research Corporation. The story …. The review ….

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The Physics of Charge-Asymmetric Molecular States

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  1. The Physics of Charge-Asymmetric Molecular States Why I never let go of my Ph.D. thesis research! Rhodes Scholars Symposium University of Illinois, Chicago March 28, 2012 Supported by: National Science Foundation Research Corporation

  2. The story …

  3. The review …

  4. Major result: Inner-shell ionization • Common assumption – only the least bound electron is ionized by tunneling in a strong field and the resulting ion is left in the ground state. • Our (Gibson, Rhodes, et al.) result showed inner-shell ionization and, consequently, excitation of the ion by the strong laser field. In fact, excitation led to fluorescence of a previously unobserved state of N22+. • Results met with some resistance! • I continued to pursue this question in different ways as a postdoc and a professor.

  5. Postdoc work at Bell Labs Could ionize the 1πu and 2σg electrons, as well.

  6. Dissociation Channels:  N2+ + N0+ (15.1 eV)  N3+ + N1+ (17.8 eV)  N4+ + N2+ (30.1 eV) • N2  N21+ N22+ N1+ + N1+ N23+ N1+ + N2+N24+ N2+ + N2+N25+ N3+ + N2+N26+ N3+ + N3+ N27+ N4+ + N3+

  7. Conclusions from VUV SpectraCoffee and Gibson, PRA 69 (2004) • Nitrogen shows many fluorescence lines generated from direct strong field excitation. • In all cases, the excitation involves one or two 2s holes. • Some upper states consist of multiply excited states. One is at 25 eV above the ground state. N2+: 2s2p2 – 2p3. • Direct lines identified from N4+ - a state not seen in ion TOF data, until recently.

  8. Theory of Multiphoton Coupling in Molecules [PRL 89 263001, PRA 67 043401] • Atoms do not show signs of multiphoton excitation when exposed to strong laser fields: at intensities high enough to drive multiphoton transitions, the ac Stark shift detunes the laser and ionization sets in. • So, what is so special about ionized diatomic molecules? • They have an excited state structure that is highly susceptible to multiphoton coupling.

  9. 2 electrons in a double well. 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.

  10. 3-Level Model System This system can be solved exactly for the n-photon Rabi frequency!

  11. N-photon Rabi Frequency: 2-level frequency from Duvall (or Shirley), et al.: In the 3-level system, multiphoton coupling depends on R23 while the AC Stark shift depends on R12. In the 2-level system, both effects come from the same coupling.

  12. Perfect Floquet Ladder of States: The pair of ionic 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.

  13. Example: Population transfer in a model system: A24+.

  14. Again, a Floquet Ladder of States: The pair of strongly coupled ionic states is so effective, it can assist a high-order multiphoton transition to a regular covalent state! Verified through a 5-level calculation. Transition requires R23 to be large.

  15. Can even get adiabatic transfer on a 10-photon transition!

  16. Pump-probe experiments in I2

  17. Iodine potential curves Many time-resolved pump-probe experiments are possible. Right now, we are specifically interested in the I2+ + I0+ states. The (2,0) and (1,1) curves form an excimer-type system in the dication! (2,0) is strictly bound while the (1,1) is at best quasi-bound. Wanted to see if we could populate the (2,0) states.

  18. Populating the (2,0) state:

  19. pump-probe delay=180 fs Simulation: trapped population in the (2,0) potential well The (2,0) potential curve measured from the A state of I2+ in our previous work: PRA 73, 023418 (2006)

  20. Asymmetric channels can show spatial asymmetry in a 12 field • An asymmetric channel like (2,0) actually consists of two states with gerade and ungerade symmetry. Then one can form:(2,0)R ~ (2,0)g + (2,0)u(2,0)L ~ (2,0)g - (2,0)uwhere R and L refer to the 2+ ion going to the right or the left. • Of course, the (2,0)g and (2,0)u states must be populated coherently.

  21. I2+ TOF Region with 1ω2ω fields

  22. Experimental results

  23. 1-D 2-electron model From the asymmetry measurements, we can show that the ionization projects the molecules into the field-induced states. This has not really been considered before and suggests a new form of strong-field control.

  24. Conclusions • Strong laser fields do a lot more than just ionize the least bound electron and leave the ion in its ground state. • Diatomic molecules have a structure that is highly susceptible to strong field excitation. • High levels of excitation are seen through the dissociation channels and direct fluorescence from the excited molecule. • Ionization occurs within the electronic structure induced by the strong laser field.

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