Mass Spectrometric Studies of Pulsed Laser Ablation: Existence of Rydberg State Atoms
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Gas Inlet /e
/
t
Laser Pulse
Plat
Ioi e Ionizer
Deflection Plates
Ablation Chamber
Detection Chamber
Figure 1. TOFQMS apparatus (not to scale). The targetdetector distance was 0.85 m and the target-deflection plate distance was 0.50 m. The gas inlet was not used.
He mixture [6]. The ablated material traveled down a time-of-flight (TOF) tube toward the quadrupole and detector. The TOF tube was differentially pumped so that the mass spectrometer could be maintained at a low pressure (-5x10- 8 Torr). The results of the physical and chemical interactions of the plume with a background ambient will be presented elsewhere; data presented herein were collected in vacuum.
About 50 cm downstream from the ablation region, a set of electrostatic deflection plates was located to separate the charged particles from the neutrals. The plates were aligned parallel to the flight path and were 1.0 cm apart and 10.0 cm long. It is believed (discussed below) that the plasma density was sufficiently low here for electrostatic separation to occur. The ablated vapor first passed through two orifices that were 6.0 and 3.0 mm in diameter, and located at 7.5 and 45 cm along the flight path, respectively. The latter orifice was at the entrance of the deflection plates. The Debye length, D, a characteristic distance for shielding in plasmas, is given by
D = 740(T%)"
(1)
where D is in cm, T is the plasma temperature in eV, and n is approximately the plasma (ion) density in cm- 3 . At the target surface, for T = 0.2 eV (- 2200K) and for an ion density of about 1012-1013 cm- 3 (about 10% of a typical plume density above threshold fluence), D is calculated to be about 50 gin. At the electrostatic plates downstream, an exact value for the plasma density under the present experimental geometry was difficult to estimate, but a variety of factors including off-axis sampling (= 1-2 mm below the laser focus), low fluence measurements (at or near threshold), and the presence of the orifices help to ensure that Debye shielding was not occurring downstream. It is estimated that the Debye lengths were on the order of 1 mm or greater. As will be shown below, the ability to sweep out the low energy ions was taken as evidence that Debye shielding at the electrostatic plates is unlikely. At the entrance of the quadrupole, an electron impact ionizer was used to ionize neutral species. When monitoring ions, the ionizer was turned off and the electrostatic deflection plates grounded. Electric fields up to 6 kV/cm could be applied but typically, only several hundred volts/cm were used. These potentials were sufficient to field ionize highly excited Rydberg state atoms [11, 12]. Time-of-flight (TOF) profiles were collected using a multichannel scaler interfaced to a computer. Data collection was synchronized to the firing of the laser. By knowing the total flight distance (about 0.85 in), and the mass, transformation of the TOF profiles to either velocity or energy profiles is straightforward. Typical TOF profiles represented a sum of 500-1000 las
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