The application of electrostatic lenses is demonstrated to give a substantial improvement of the two-dimensional (2D) ion/electron imaging technique. This combination of ion lens optics and 2D detection makes “velocity map imaging” possible, i.e., all particles with the same initial velocity vector are mapped onto the same point on the detector. Whereas the more common application of grid electrodes leads to transmission reduction, severe trajectory deflections and blurring due to the non-point source geometry, these problems are avoided with open lens electrodes. A three-plate assembly with aperture electrodes has been tested and its properties are compared with those of grid electrodes. The photodissociation processes occurring in molecular oxygen following the two-photon 3dπ(3Σ1g −)(v=2, N=2)←X(3Σg −) Rydberg excitation around 225 nm are presented here to show the improvement in spatial resolution in the ion and electron images. Simulated trajectory calculations show good agreement with experiment and ...

Velocity map imaging of ions and electrons using electrostatic lenses: Application in photoelectron and photofragment ion imaging of molecular oxygen

(b) Write out the equations for the time dependence of ρ11, ρ22, ρ12 and ρ21 assuming that a light field interaction V = -μ12Ee iωt + c.c. couples only levels |1> and |2>, and that the excited levels exhibit spontaneous decay. (8 marks) (c) Under steady-state conditions, find the ratio of populations in states |2> and |3>. (3 marks) (d) Find the slowly varying amplitude ̃ ρ 12 of the polarization ρ12 = ̃ ρ 12e iωt . (6 marks) (e) In the limiting case that no decay is possible from intermediate level |3>, what is the ground state population ρ11(∞)? (2 marks) 2. (15 marks total) In a 2-level atom system subjected to a strong field, dressed states are created in the form |D1(n)> = sin θ |1,n> + cos θ |2,n-1> |D2(n)> = cos θ |1,n> sin θ |2,n-1>

The Quantum Theory of Light

Publisher Summary This chapter describes the coherent population trapping in laser spectroscopy. Coherent population trapping may be also described as the pumping of the atomic system in a particular state, the coherent superposition of the atomic states, which is a nonabsorbing state. The exciting radiation creates an atomic coherence, such that the atom's evolution is prepared exactly out of phase with the incoming radiation and no absorption takes place. The chapter discusses the basic properties of an atomic system prepared with the coherent population-trapping superposition of states and outlines experimental observations concerned with the establishment of coherent trapping in different discrete systems. The chapter also discusses the theoretical and experimental aspects of trapping that involve states of the continuum and reviews the theoretical and experimental features associated with coherent population trapping in laser cooling, adiabatic transfer, lasing without inversion, pulse matching, and photon statistics. The theoretical aspect of coherent population trapping created by spontaneous emission is also discussed in the chapter.

Coherent Population Trapping in Laser Spectroscopy

1. Understanding chemical reactions at the molecular level 2. Molecular collisions 3. Introduction to reactive molecular collisions 4. Scattering as a probe of collision dynamics 5. Introduction to polyatomic dynamics 6. Structural considerations in the calculation of reaction rates 7. Photoselective chemistry: access to the transition state region 8. Chemistry in real time 9. State-changing collisions: molecular energy transfer 10. Stereodynamics 11. Dynamics in the condensed phase 12. Dynamics of gas-surface interactions and reactions.

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Molecular reaction dynamics

We review some basic techniques for laser-induced adiabatic population transfer between discrete quantum states in atoms and molecules.

Laser-induced population transfer by adiabatic passage techniques.

A technique that produces significant alignment of molecules in a beam should aid a wide range of experiments geared towards understanding and controlling molecular processes in the gas phase. Manipulation of the molecular-axis distribution is an important ingredient in experiments aimed at understanding and controlling molecular processes1,2,3,4,5,6. Samples of aligned or oriented molecules can be obtained following the interaction with an intense laser field7,8,9, enabling experiments in the molecular rather than the laboratory frame10,11,12. However, the degree of impulsive molecular orientation and alignment that can be achieved using a single laser field is limited13 and crucially depends on the initial states, which are thermally populated. Here we report the successful demonstration of a new technique for laser-field-free orientation and alignment of molecules that combines an electrostatic field, non-resonant femtosecond laser excitation14 and the preparation of state-selected molecules using a hexapole2. As a unique quantum-mechanical wavepacket is formed, a large degree of orientation and alignment is observed both during and after the femtosecond laser pulse, which is even further increased (to 〈cosθ〉=−0.74 and 〈cos2θ〉=0.82, respectively) by tailoring the shape of the femtosecond laser pulse. This work should enable new applications such as the study of reaction dynamics or collision experiments in the molecular frame, and orbital tomography11 of heteronuclear molecules.

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Impulsive orientation and alignment of quantum-state-selected NO molecules

Molecules adsorbed on a surface are known to show preferential orientations, and in particular, the interaction potential between a linear molecule and a surface depends on the orientation of the molecular axis. But the fact that the molecules eventually adsorbed are orientated with respect to the surface is not evidence that the dynamics of gas–surface interactions is governed by the initial molecular orientation in the gas phase. For example at very low velocities the molecule might achieve its optimum orientation adiabatically during its approach to the surface. Dependence of scattering dynamics on molecular orientation can also be degraded as a result of surface structure and surface vibrations. There have been, experimental studies, however, which suggest that orientational or steric effects could influence gas–surface scattering1–4. Thus, the much larger rotational excitation for NO- than for N2-scattering from Ag(lll) indicates that large anisotropies can occur in a potential for gas–surface dynamics (G. O. Sitz et al. manuscript in preparation). The double rotational rainbow observed in the rotational state distribution for NO/Ag(111) (refs 3 and 4) can also be interpreted as a manifestation of this anisotropy5–9. These studies all indicate that steric effects could be important and here we report on scattering of orientated NO from Ag(111) to provide direct experimental evidence for the importance of such effects in gas–surface interactions.

Observation of steric effects in gas–surface scattering

Coherent control of molecular orientation

Photofragment imaging is used to study the internal state and velocity distributions of methyl fragments following photodissociation of CD3I molecules by 266-nm radiation. The molecular fragments are state selectively ionized via 2 + 1 resonance enhanced multiphoton ionization (REMPI) through the 3pz Rydberg state of methyl. The photofragment imaging technique,1,2 which records the entire velocity distribution for the state selectively ionized fragment, allows us to record the velocity distribution of the ionized photofragments for every laser pulse. Because the methyl fragment is state selectively ionized by the REMPI process, and the parent methyl iodide molecule is cooled in a molecular beam, the velocity of the ion mirrors the internal state distribution of the iodine atom born in coincidence with the selected state of the methyl. For a selected state of the methyl radical two velocities are seen. The faster fragments are bom in coincidence with ground state iodine l(2P3/2), and the slower fragments are born in coincidence with excited state iodine I(2P1/2). The intensity ratio of fast to slow methyl fragments is then unique to each internal state of the methyl fragment and is used to help assign the methyl REMPI spectrum. This is illustrated in the spectra shown in Fig. 1 in which there are two overlapping bands.

Photofragment imaging: the 266-nm photodissociation of CD3I

The state of the art of reactive scattering with oriented molecules is presented. The alignment and orientation achieved with laser methods and deflection fields are compared to each other. They can be described very well with a short Legendre series. The steric reactive scattering phenomena turn out to be large and are closely connected to the configuration of the transition state. In a few cases a deconvolution to a steric reaction probability function can be carried out. The concept of azimuthal steric dependence is introduced.

Steven Stolte

Papers

Impulsive orientation and alignment of quantum-state-selected NO molecules

Observation of steric effects in gas–surface scattering

Coherent control of molecular orientation

Photofragment imaging: the 266-nm photodissociation of CD3I

Reactive Scattering Studies on Oriented Molecules