APLICACIONES Y PROCESOS CRÍTICOS DE NEGOCIO

Va S w a g elo k m ale pipe weld con n ector

Va S w a g elo k nut

Internal weld

Figure 2.5. %" Swagelok to 70 mm confiât conversion piece.

2 3 Femtosecond laser

In the present work, laser pulses of 60 fs have been used to field-ionize a range of triatomie moleeules and atomic gases. The laser system uses ehirped-pulse amplification (Strickland and Mourou 1985) of 50 fs pulses from a titanium: sapphire (Ti:S) oscillator.

2.3.1 The 50 femtosecond laser system

Figure 2.6 shows a diagram of the laser system used in the present work. A Spectra Physics BeamLok 171 argon-ion laser pumps a Spectra Physics 3960C Ti:S oscillator, producing a pulse energy of around 10 nJ in a pulse duration, i = 50 fs at a wavelength of ^ = 790 nm. The oscillator generates these short pulses at a frequency of 82 MHz. It is necessary to use chirped-pulse amplification (CPA) in the case of such short pulses, as it is impossible to generate the pulse energies necessary for these experiments without destroying the pulse quality. An all-reflective stretcher, comprising of two 1500 lines/mm gratings (G1 and G2 on figure 2.6) and a concave mirror (CM) with a radius of curvature (ROC) of 730 mm, is used to increase the pulse duration to 180 ps. The oscillator is isolated from reflections by a Faraday rotator (FI on figure 2.6) that also switches the stretched pulse into the amplifier by rejecting it perpendicular to the oscillator pulse. The stretched pulses are amplified in a multi-pass confocal amplifier containing a Ti:S crystal (Crystal Systems, figure-of-

merit = 150 and a su = 4.8), pumped with 67 m j from a Spectra Physics GCR 270-10

Nd: Y AG laser, with a pulse length o f 8 ns. The stretched pulses pass through the

crystal five times, controlled by four spherical mirrors, each with a radius of curvature of 2000 mm. On figure 2.6, these mirrors are indicated by ‘S’. The

T e le s c o p e A u to ­ c o r r e la to r CM C o m p r e s s o r —\ 1W34 T i;S --- amplifier crystal G2 P _G1 S tr e tc h e r R oof prism To TOFMS N d :Y A G 800 m J 532 n m

Figure 2.6. Schematic o f the femtosecond laser system at the Rutherford Appleton Laboratory. The elements o f the laser are labelled thus: Faraday Isolator (FI), concave mirror (CM), spherical mirror (M), Pockel's Cell (PC) and gratings (G1 - G4).

preamplified pulse train is then removed from the amplifier and passed through a Pockel’s Cell (PC in figure 2.6). Pulses are selected from the 82 Mhz pulse train at a repetition rate o f 10 Hz. The selected pulses are then re-injected into the amplifier for another five passes through the Ti:S crystal. The whole amplification process raises the pulse energy to 4 mJ in a pulse o f 180 ps. The amplified stretched pulses then enter a compressor, consisting of a pair of 1500 lines/mm gratings in parallel arrangement (G3 and G4 in figure 2.6). This compresses the pulses back to x = 50 fs, with a pulse energy of 1 mJ. Optical losses are unavoidable in the compression process, mainly due to degradation of the reflectance of the gratings. The output pulse shape is monitored on a scanning autocorrelator, which receives the output of a

1 0% beam-splitter placed in the path of the oscillator pulse.

2.3,2 Beam transport and monitoring

Figure 2.7 is a schematic of the optics used to transport o f the laser pulses to the spectrometer. The mirrors used throughout the present work are all gold-film on a glass substrate, which have the highest reflectivity for 790 nm laser light (-93%). To minimise optical losses, the number of mirrors is kept to a minimum.

Following compression, the laser pulses have linear polarization, with the electric field vector vertical, parallel to the axis o f the TOFMS drift tube. However, the beam leaves the compressor 20 cm above the optical bench. A periscope is used to lower

G P Xil W

- T o D S O

Figure 2.7. Top-down schematic o f the beam transport optics. The elements present are: periscope (P), gold mirrors (M l - M5), apertures (A), glass plate (GP), Integrating sphere (IS), photodiode (PD)„ half-wave plate QJ2) and entrance window (W).

Figure 2.8. The periscope used to drop the beam from the exit o f the compressor to the height o f the TOFMS.

the beam as shown in figure 2.8. The arrangement of the mirrors in the periseope flip the polarisation direction through 90°. This shall be referred to as ‘perpendicular’ polarization, as the electric field vector is perpendicular to the TOFMS axis.

A half-wave plate, mounted in a motorized rotation stage, controls the polarization direction. This component is indicated by XJ2 in figure 2.7. A quarter-wave plate can

be inserted into the beam-path after the half-wave plate to produce circular

polarization.

Following optical losses from the mirrors and optical components, 900 pJ per pulse is available at the entrance window. The laser pulses enter the vacuum system through a fused-silica window (indicated by W on figure 2.1), which is mounted on one of the 70 mm confiât ports on the vacuum chamber. The path of the laser pulses is perpendicular to the axis of the TOFMS. On transmission though the entrance window, we estimate a 10% temporal dispersion, hence the pulse length is around 55 fs in vacuum.

Pulse-to-pulse fluctuations in laser energy are monitored on a photodiode (PD). A glass plate, indicated by GP in figure 2.7, is inserted into the beam path, which transmits the majority (98%) of the pulse. The reflection generated by the glass plate is then passed through an integrating sphere whieh insures that the photodiode response is independent of polarisation direction. Typically, the laser pulse energy fluctuates by of the order of 50%. The signal from the PD is used to produce a trigger for the DSO, provided the PD signal is within ±10% of the user defined level

(as will be discussed in section 3.5).

Typically, with the PD unsaturated and with a laser pulse energy o f 900jiJ entering the TOFMS, the unsaturated PD signal had an amplitude of the order of 200 mV, with a full-width, half-maximum of 30 ns.

In a subsidiary experiment, the PD signal was measured with respect to the laser pulse energy (as measured with a calibrated pyrometer) directly in front o f the TOFMS. It was found that the PD response was directly proportional to the laser pulse energy.

2.3.3 Beam focussing in vacuum

The laser pulses are focussed into the source region of the TOFMS, either by a parabolic mirror with a focal length of 2 . 2 cm, or by a spherical mirror with a focal

length of 5 cm. The parabolic mirror is a glass substrate coated front and back with aluminum, mounted on a PFTE block, which is screwed directly to the bottom-plate o f the TOFMS. The mirror is held onto the PTFE mount by way of two phosphor- bronze springs. Each of these springs is attached using two stainless steel M l.5 screws. A fine gold wire is attached to the back face of the mirror and to one o f the phosphor-bronze springs, which is attached to earth to prevent electrostatic charging. Figure 2.9 is a schematic of the parabolic mirror and mount.

S id e v ie w P h o s p h o r b r o n z e s p r in g S / S M1.5 s c r e w M irro r P T F E s u p p o r t B o tto m p l a t e S / S M3 s c r e w

o

V

y

The spherical mirror is a glass substrate with an aluminum film deposited on a eoneave faee. The mirror is mounted in a threaded stainless steel fitting, whieh is shown in figure 2.10. This mount is screwed directly into the cradle, and the position of the mirror adjusted until the focal spot is central to the axis of the spectrometer.

S /S cradle

M6 mounting screw

S /S mount S /S lens holder S /S lens retaining ring PTFE sp acer

LASER IN