Atomic and Molecular Physics Laboratory

University of  Ioannina - Department of Physics

Research

Typical experimental results

(a) Electron spectrum resulting from two-photon ionization of ground state Mg at λ_UV~293 nm exhibiting a single peak The inset shows a simplified energy level diagram of the process. (b) Electron spectrum resulting from the interaction of ground state Mg with the fundamental laser radiation (λ_VIS~590 nm).

Multiphoton ionization via autoionizing states of alkaline earth atoms

This set up is used for the study of multiphoton ionization of alkaline earth atoms via autoionizing states. Two cases were studied, the two- and four– photon excitation of the mp^{2 1}S₀ states (Mg - m=3, Sr - m=5).

The laser in use is a Nd: ΥAG pumped dye laser operating in the 561-596 nm wavelength range for the case of Mg and in the 715-737 nm wavelength range for the case of Sr, offering wavelength calibration within ~0.01Å. For the two-photon ionization studies the fundamental frequency is doubled by a BBO type-III crystal. The system operates at a repetition rate of 10 Hz and it delivers linearly polarized pulses of ~5 ns duration. The earth alkali vapor is produced in an electrically heated stainless steel oven mounted at the top of a laser-atom interaction chamber and connected to it through a water-cooled baffle. The atomic beam interacts at right angle with the laser beam. The ions and electrons created by the laser-atom interaction are detected perpendicularly to both beams. The photoelectrons are energy-analyzed by an electrostatic 166^o spherical sector electron energy analyzer equipped with an electrostatic lens The latter are detected by a MCP detector mounted at the exit of the analyzer. The system is placed within a μ-metal box for protection against parasitic magnetic fields. The achieved energy resolution ΔΕ_FWHM/E_o is ~0.8% for transmission energies E_o in the range 10-30 eV. The angular resolution is found to be ~3^o with the electrostatic lens turned-off and ~4^o when the lens is turned-on.

Time-Resolved fs Mass Spectrometry

The collision chamber

The Hemispherical Deflector Analyser

Molecular Dynamics under strong asymmetric (ω/2ω) laser fields

Photoionization Microscopy of Quasi-bound Electronic States

Time-Resolved (ns) UV-VIS florescence spectrometry

Z-scan for non-linear properties of matter

Molecular fragmentation under broadband fs laser pulses

Time-resolved fs mass spectrometry utilizing  a single photon molecular excitation followed by single or multi-photon ionization in a pump-probe scheme is currently developed in order to investigate molecular dynamics.  The single photon involved in the pump and probe steps is expected to lead to much clearer results as opposed to a multi-photon per step scheme.

The fs laser beam is split in two parts, the most intense of which is used to produce higher order harmonics in a static gas cell (pump). Currently, the 5th harmonic (λ=160 nm) is selected. The less intense laser beam (probe)  is merged with the pump beam collinearly on a hollow mirror. Both beams  are guided to the interaction chamber where they are focused by a spherical mirror on either a pulsed jet or effusive beam molecular target after they have been spatially and temporally overlapped. The temporal control of the beam is performed on the probe beam via a delay stage.  The ionic fragments are detected via a TOF mass spectrometer.

Currently, we are optimizing the setup incorporating automated data acquisition approaches. Preliminary data are recorded on a daily basis aiming at the investigation of the dynamics in dark states, isomerization, etc, of aromatic molecules.        

In this approach, we exploit few-cycle laser pulses to systematically investigate the dynamics of fundamental molecular dissociation processes.  The fragmentation channels are monitored as a function of the laser pulses duration which is controlled through their imposed positive chirp. The experiments involve a low-field TOF mass spectrometer fully developed and optimized in CLF-UoI. The experiments are performed at the Centre for Plasma Physics and Laser in Crete using the 7-fs CEP stabilized laser facility. Currently we are focusing our efforts on simple molecules like H₂ and D₂ and the dissociation processes of Bond Softening,  Above Threshold Dissociation and Charge Resonance Enhanced Ionization. Future research campaigns involve more complex molecules (e.g. aromatic ) and paths.  Our experimental results are supported by ab initio calculations performed by Dr. L.A.A. Nikolopoulos (NCPST-DCU)

 

 

Comparative H₂⁺ dissociation yields at different laser pulse durations. All data correspond to equal H₂⁺  ionization yield. The dashed  blue line corresponds to 7-fs pulses and the solid red line to all the other pulse durations

1sσg and 2pσu potential-energy curves of H₂⁺ along with their dressed diabatic companions in the Floquet picture. The vibrational states of the bound 1sσg state along with the  BS (1ω) and ATD (2ω) dissociation channels are depicted as well.

           General overview of the experimental setup.

Preliminary experimental results:  Ionization of  toluene in a 5ω + ω  pump-probe scheme.

The experimental setup

Laser fields with asymmetric profiles has attracted widespread interested in the past few decades. It has been proven, both theoretically and experimentally, to be useful tools in order to manipulate the orientation of a gas phase molecular sample. Towards this direction a lot of effort has been made because an orientated molecular sample can be utilized in various applications, i.e. in high harmonic generation, chemical reaction dynamics etc. An asymmetric laser pulse can be produced by the superposition of the fundamental laser frequency and its second harmonic. Its mathematical description is given by the equation:

 

 

where E_o(t) is the pulse envelope, γ the relative electric field strengths and φ the phase difference between the two colors. Experimentally, such a pulse can be produced with little effort and its asymmetric profile can be easily manipulated simply by adjusting its phase φ or γ parameter.

 

Within the context of the MO-ADK and SFA ionization models, it is well understood that when a strong ultrafast laser field interacts with a gas phase molecular sample, the molecules that are more likely to ionize/dissociate are those that are already oriented towards its polarization vector. Based on this argument, it is expected that under a 30 fs ω/2ω laser pulse the produced fragments should exhibit directionalities that can be altered simply by adjusting the phase φ of the ω/2ω field. Several small molecules (O₂, CO, N₂, N₂O, CH₃I) have been investigated under the ω/2ω laser field by means of time of flight mass spectrometry. The resulting directionalities are apparent in the time-of-fight mass spectrum, as well as in the integrated signal of each peak.

The experimental setup for tailoring the ω/2ω laser field.

Top: TOF mass spectrum depicting the N₂O fragmentation for two different ω/2ω phases.

 

 

Left: Modulation (beta - β  parameter) of the NO⁺ and NO²⁺ fragments of the N₂O fragmentation as a function of the  ω/2ω phase.

The aim of Photoionization Microscopy (PM) may be understood on the basis of hydrogenic LoSurdo-Stark effect. The Hamiltonian of a hydrogen atom in a uniform static electric field of strength F directed along the z-axis, is separable in parabolic coordinates ξ=r+z, η=r–z and φ=arctan(y/x). Separability along φ leads to the azimuthal quantum number m, while separability along ξ and η leads to parabolic quantum numbers n₁ and n₂, counting the nodes in the ξ- and η-part of the wave function respectively. The electron is always bound along the ξ–coordinate. The energy range of interest here is located between the classical saddle point energy E_sp = –2·F^1/2 au and the field-free ionization limit E=0. For E ³ E_sp the electron escapes in the negative z-direction, along the η-coordinate. The electron escapes either via tunneling or over the η–potential barrier. Within E_sp ≤ E ≤0 quasi-bound and continuum Stark states with different n₁ values coexist. Despite their coexistence, all Stark states of the hydrogen atom are orthogonal to each other. Wave function microscopy aims at recording the photocurrent density j(ξ,η) at a macroscopic distance, along a given constant η=η_ο paraboloid. In the absence of resonances, it may be written as

                                                                                                              

where χ_1,n1 denotes wave functions along the ξ-coordinate. The coherent superposition  has the form of an interferogram.

Principle of photoionization Microscopy: The electron flux stemming from the photoionization of an atom in the presence of a static electric field is recorded perpendicularly to the field and at macroscopic distance, i.e. approximately along a given constant η =η_ο paraboloid. The image corresponds to the wave function along the ξ-coordinate where the electron motion is always bounded (in the example shown, F≈1 kV/cm and the electron excitation energy exceeds E_sp by 10 meV). Depending on the electron energy with respect to the maximum of the η-potential, the classical electron motion may be either bounded (a) or free (b). In the former case the electron can escape solely via tunneling and the image corresponds to a direct macroscopic projection of a quantum standing wave characterizing the quasi-bound electronic state, where the electron is initially localized within the inner η-potential well.

The photoionization microscope. It is based on a standard velocity map imaging designed and it is equipped with an electrostatic einzel lens, via which a ~20-fold magnification is achieved.

Recorded (a) radial distributions and (b) images of Li atom (m=0, F≈1 kV/cm) as a function of ε=E/|E_sp|. The ε=-0.975 image corresponds to a =2 quasi-bound state, while all other images, below and above it, to continuum states with =1. The broadening of the outer ring (in a form of a “halo”) is a sign of electron tunnelling through the barrier.

The z-scan technique is nowadays customarily employed for measuring the non-linear optical properties of transparent or partially transparent materials. In a typical z-scan measurement, a focused nearly Gaussian laser beam hits the sample under study perpendicularly (Fig. 1). The sample is scanned along the beam propagation axis, z, in the vicinity of its waist (focal point) and experiences a radially varying laser beam intensity profile, I(z,r). Far from focus, the absorption coefficient and refractive index of the film acquire their conventional, intensity-independent values, say a_lin and n_o, respectively. Near the waist of the beam, however, absorption and refraction are modified and become intensity dependent, a(I) and n(I)= n_o+n₂∙I. The value of n₂ as well as the exact form of a(I) depend on the characteristics of the sample under study and the laser beam used. Ultra short-pulse laser operation reveals electronic nonlinearities, while continuous wave (cw) operation or a long interaction time reveals nonlinearities of thermal origin.

 

Our primary purpose here was the study of the nonlinear optical characteristics of GafChromic^® docimetry films (if any). These films are widely used nowadays for measuring dose distributions of ionizing radiation in various applications and, in particular, in every day medical practice. They contain an active layer of disubstituted diacetylenes (R-CºC-CºC-R', where R and R' are monovalent organic substitutes). Upon irradiation this active layer undergoes dose-dependent polymerization in the, so-called, blue polydiacetylene (PDA) form, exhibiting two main absorption bands at ~615 and ~670 nm. Consequently, they change color upon irradiation from light clear blue to darker blue depending on the absorbed dose. Dose assessment is achieved by exploiting the dose-dependence of the linear absorbance A=a_linL (with L the length of the absorbing medium) and often carried out using light sources of appreciable red spectral content. A secondary purpose of our work was the exploration of non-linear dose-measuring techniques.

Set-up for the z-scan measurements. He/Ne: Helium/Neon cw laser (λ=633 nm), maximum power ~10 mW, linearly polarized, Ch: chopper, VNDF: variable neutral density filter (rotatable), BS: beam splitter, PM: power meter, L: lens, F: film, RB: rotatable base (with respect to the beam propagation axis, z), SM: stepper motor, CBS: cube beam splitter, AL: auxiliary lens, Phd 1 and 2: photodiodes, OSC: osciloscope, PC: personal computer.