The Dynamics Of Molecules In Intense Ultrashort Laser Fields: Measurements of Ultrashort, Intense Laser-induced Fragmentation of The Simplest Molecular Ion (H2+ )

Necati Kaya

H2+ the most elementary molecule in natureTheoretically, H2+is usually treated as a two-level system, since its ground 1sgand the first excited state 2puare well separated from next higher states.This simple molecular structure makes possible accurate fundamental quantum mechanical calculations.Contrary to its theoretical simplicity, the preparation of H2+is not easy and therefore the experiments on H2+are rather rare.

Why H2+ ?H2+

Photodissociation of H2+Potential Energy Curves of the two lowest states of H2+ in a weak field.Images from http://www.mpq.mpg.de/~haensch/h2+/introduction.htm

At intensities higher than 1012W/cm2the coupling between the ground 1sgand the first excited state 2pubecomes very strong. These intensity regimes can be characterized with the Rabi frequency R (see [1,2]), which measures the strength of the radiative coupling:

Potential Energy Curves of the photon dressed states In this regime molecule-light system is usually described by potential curves "dressed" with photons or with so-called light-induced potential curves.Images from http://www.mpq.mpg.de/~haensch/h2+/introduction.htm

Potential seen by the ionizing electron.http://www.mpq.mpg.de/~haensch/h2+/images/Coulomb%20potential.gif

(Sayler, 2008)Hamiltonian H can be written as:dipole moment of the moleculeMolecular reduced massElectricfield due to the lasernuclear kinetic energypotential due to the laser fieldthe sum of the electronic kinetic energy and Coulomb interaction of all particlesFloquet picture

The Floquet picture allows one to envision the effects of the laser-induced adiabatic coupling of Potantial Energy Curves, which are obscured in the Vertical Transition and diabatic Floquet pictures. The Floquet theory is a useful tool when dealing with laser-molecule interactions, which allows one to easily make qualitative predictions about the behavior of molecules in a laser field. Furthermore, this picture will be referred to in upcoming discussions of experimental results as a basis for expectations and results. However, one must remember the assumptions made in generating the Floquet potentials so that the theory is not over extended.

H2+ adiabatic and diabatic Floquet potential energy curves. (a) Using vertical arrows to represent photon absorption/emission. (b) Floquet picture with diabatic curves in black and adiabatic curves in color. (c) Adiabatic Floquet curves at the 1-photon crossing with bond softening and vibrational trapping marked. (d) Adiabatic curves a the 3-photon crossing with the 1-photon emission crossing circled. Note that the vertical arrows to the right of figures mark the expected kinetic energy release (KER) for the respective processes. (Sayler, 2008)

(Pavicic, 2004) Diabatic Picture Crossings Resonant positions Adiabatic Picture Coupling Avoided crossings

Experimental Method

Simplified Diagram of Experimental setup Y. Lee MS Thesis ,2006

Ion source

Einzel Lens Dimensions (End cylinders at ground)D = 1.86 auL = 2.44 auG = 0.184 auS = 1.72 au

Mass selectionAfter extraction from the ion source, molecular ions are directed into a sector magnet (SM) through entrance slit A1 (width of 5 mm) by means of a set of horizontal and vertical electrostatic deflection plates (DP1) and an Einzel lens (EL1) (see exp setup on pg 11). By adjusting the magnetic field B of the magnet, the molecular ions of mass mm and charge q are deflected by 90 to pass through exit slit A2 (width of 5 mm).The ions selected in this way satisfy the relationIn order to select the desired molecular ion, the current after the mass selection was recorded as a function of the voltage on the magnet, which is proportional to the magnetic field B.

Delay-Line Detector

The vibrational energy v=9 is approximately 1200 cm1 (0.3 eV), with the vibrational period of 29fs. (Pavivic 2004)Typical Energy & Times Scalee- 10 eV ~ 100 asVibration 0.1 eV ~ 10 fsRotation 0.001 eV ~ 1 ps14fs 29fs Adiabatic climbing of vibrational ladders of H2+

Adiabatic climbing of vibrational ladders using Raman transitions with a chirped pump laser (Chelkowski and Gibson 1995)Linkage diagram for vibrational-ladder climbing by a pair of pulses (Raman chirped adiabatic passage scheme). Vitanov et. Al. Annu. Rev. Phys. Chem. 2001.

The pump-laser pulse ionizes the neutral H2 molecule, launching a nuclear vibrational wave packet in the H2+ ion. A subsequent control pulse modifies the vibrational-state distribution of the ion by inducing Raman transitions between the 1sg and 2pu electronic states at a given delay time. Finally, the vibrational state is probed destructively by dissociation or Coulomb explosion in the probe-laser pulse.a control-laser pulse for the lowest 5 vib. levels of H2+800 nm, 1014 W/cm2 @ 6fsLaser-controlled vibrational heating and cooling of oriented H2+ molecules (Niederhausen et al 2012)

(Niederhausen et al 2012)

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