Pulsed-field ionization hydrogen

In field ionization, hydrogen molecules near the tip region are attracted to the tip surface. They either hop around the tip surface or are field adsorbed on it. As the hopping motion and the field adsorption are dynamical phenomena, some of the ionic species detected may also come from field adsorbed states, not necessarily just from the gas phase. On the other hand, in pulsed-laser stimulated field desorption, where gas pressure is very low, of only 1 X 10-8 Torr, gas molecules are thermally desorbed by laser pulses from their field adsorbed and chemisorbed states. When they pass across the field ionization zone some of them are field ionized. The critical ion energy deficit in pulsed-laser stimulated field desorption of a gas is therefore found to be identical to that found in field ionization. In both pulsed-laser stimulated field desorption and field ionization of hydrogen, the majority of ions detected are H3 and H+. 

Early field ion emission studies of gas-surface interactions use field ionization mass spectrometry. Gas molecules are supplied continuously to the tip surface by a polarization force and by the hopping motion of the molecules on the tip surface and along the tip shank. These molecules are subsequently field ionized. The role of the emitter surface in chemical reactions is not transparent and has not been investigated in detail. Only in recent pulsed-laser stimulated field desorption studies with atom-probes are these questions addressed in detail. We now discuss briefly a preliminary study of reaction intermediates in NH3 formation in pulsed-laser stimulated field desorption of co-adsorbed hydrogen and nitrogen,... 

Here we apply the coupled-channel method to calculate photo ionization of atomic hydrogen by short (femtosecond) laser pulses at high power densities (up to 5 X 10 " W/cm ). A classical electro-dynamical field approximates the laser/atom interaction, according to (in the Coulomb gauge)... 

There is first the fundamental of a strong infrared laser. For the sake of illustration, we have chosen the case of a Ti sapphire laser operated at coi = 1.55 eV, i.e. a>i = 0.057 a.u., which is representative of the recently operated "femtosecond" sources. We have considered intensities li around lO l W/cm, which are typical. Note that, although such intensities are very high by laboratory standards, they remain quite moderate when compared to the "atomic" intensity, Iq = 3.5 lOl W/cm, which is associated to the field strength experienced by an electron on the first Bohr orbit in hydrogen, namely Fq = 5.1 10 V/cm. At intensities li =10 W/cm the atom can experience multiphoton ionization and even ATI, as shown in Fig, 1, which displays the simulation of a photoelectron spectrum for a peak intensity 1l = 2. 10 W/crri. Here, the pulse shape is assumed to be trapezoidal with linear tum-on and turn-off durations of one laser period Ti = 110 a.u., i.e. Ti = 2.6 fs), the total duration of the pulse being 8Tl. 

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