beam path to measure the arrival times of ions, which is then converted to ion mass via Eq. (3.3), along with their relative intensities. Numerous experimental parameters may then be varied to find the optimal conditions for maximising the anion signal corresponding to the species of interest.
3.4
The Nd:YAG laser and optical parametric oscillator
Velocity-mapped photodetachment experiments require a linearly polarised, high flux, nar- row bandwidth, photon source. Linear polarisation, orthogonal to ion beam and parallel with the detector, induces cylindrical symmetry on the velocity-mapped image, which is an essential requirement of the inverse-Abel transformation employed in the analysis. Mean- while, the ion packet that reaches the interaction region (approximately cylindrical with a 2mm length and 2mm diameter), often has a low ion density - therefore a high photon flux is required to ensure sufficient photodetachment events occur in order to obtain ade- quate statistics, to reveal all of the target structure. Finally, as the experiment measures electron kinetic energies, which can be related to the energy levels in the molecule via eBE =hν−eKE, a narrow laser bandwidth is important as any spread in the laser energy hν is mapped directly into the image.
To satisfy all of the above conditions, a Continuum Powerlite Precision II 9010 Nd:YAG pulsed laser is employed. This laser operates at 10Hz, producing 5ns pulses with energies up to 2000 mJ at 1064 nm[61]. While the source side of the lab operates at 30 Hz, the imaging side (Laser/VMI/Camera) operates at 10 Hz. The source side is operated at the higher frequency, with only every third packet involved in photodetachment, as this has proven to help improve the stability of the ion source[46]. The Nd:YAG laser is seeded by a NP Photonics fibre laser to improve the stability of the pulses, with the YAG pulse shape monitored by a photodiode during operation, to ensure a good quality pump beam for the Continuum Sunlite EX OPO.
A diagram of the Nd:YAG laser used in this work is given in Fig. 3.10. Flash lamps, powered by a large capacitor bank within the laser power supply, charge the oscillator rod which lases at 1064 nm when the Q-switch is fired. The laser beam then passes through two amplifying rods to reach a pulse energy of up to ∼2000 mJ. To allow for various detachment wavelength options, the beam passes through two nonlinear KDP crystals for generation of the second (532 nm), third (355 nm), and fourth (266 nm) harmonics from the YAG fundamental (1064 nm). Dichroic mirrors are then used to clean the beam to ensure only a single wavelength is transported to the experiment.
An important feature of the Precision II 9010 laser system is the ability to externally trigger the Q-switch and flash lamp discharge times, enabling the laser system to be synchronised to the ion beam. Temporally intersecting a 5ns laser pulse with the pulsed ion packet from the beamline requires high precision in the Q-switch timing. In order to optimise the temporal overlap of the ion packet and laser pulse a Q-scan is performed, where the Q-switch is varied in regular increments while the number of photoelectron counts per shot is measured. This ensures that the laser will be fired at the correct time, to optimally intersect the ion packet from the beamline and maximise the number of events recorded.
Fibre seed laser Rear mirror Pockels cell Waveplate + polarizer Oscillator rod 2 x Amplifier rods Second harmonic crystal Third harmonic crystal Output shutters Dichroic stages
External Q-switch trigger
Figure 3.10: Nd:YAG Powerlite Precision II 9010 laser, used in this work. The oscillator rod is charged via flash lamps powered by a large capacitor bank within the laser power supply. When the Q-switch fires lasing at 1064 nm occurs, seeded by the NP photonics fibre laser. The photons are transported to two amplifying rods, producing 5ns pulses with energies up to 2000 mJ. Nonlinear KDP harmonic crystals may then be used to convert the 1064 nm photons to the second (532 nm), third (355 nm), or fourth (266 nm) harmonic.
3.4.1 Sunlite EX OPO
For cases where flexibility in the detachment wavelength is required, the third harmonic of the Nd:YAG (355 nm) is used to pump a Sunlite EX optical parametric oscillator (OPO). The OPO system is a tunable high efficiency source, capable of generating narrowband radiation in the visible and near infra-red spectral region between 445−1750 nm. Optical parametric processes involve three photon interactions, where one pump photon splits into a pair of less energetic ones. The two photons produced typically do not have the same energy, with the high energy photon refereed to as the signal and the low energy photon the idler. This is achieved in the Sunlite EX OPO by employing a nonlinear beta-barium borate (BBO) crystal with a birefringence axis. For a given angle between the crystal axis and pump beam, only one pair of signal/idler frequencies will conserve angular momentum. The signal/idler frequencies for a given phase matching angle is given in Fig. 3.12. By tuning the birefringent axis based on this calibration curve, the output photon source may be tuned to any desired frequency between 445 and 1750 nm. A narrow bandwidth (of less than 0.1 cm−1) is achieved by operating in the ExtRA ordinary plane of the crystal, as opposed to the ordinary plane, as the narrow crystal acceptance angle of the ExtRA ordinary plane restricts the bandwidth of the oscillating radiation. An image of the OPO is presented in Fig. 3.11.
The system is controlled via computer software, scanning to any desired wavelength by angular tuning the relevant crystals based on calibration tables. The exact wavelength is measured using a High Finesse WS7Super Precision wavemeter. The laser output from the EX OPO is then transported to the interaction region, using a series of optics, including Glan-Laser polarisers to ensure vertical polarisation of the laser. An additional Sunlite FX-1 UV doubling crystal may be used to produce wavelengths less than 455 nm, however
§3.4 The Nd:YAG laser and optical parametric oscillator 41
this further reduces the pulse energy (1-2 mJ, cf. 5-50 mJ) which has proved problematic.
OPO crystal Diffraction grating Tuning mirror Dichroics OPA crystals OPO output 355nm YAG output
(a) Sunlite EX OPO used in this work. The 355 nm output of the Nd:YAG is tuned to the desired frequency via a parametric 3- photon process using nonlinear BBO crys- tals. The input beam is split by a beam splitter, with one arm going through the os- cillator crystal, tuning mirror, and diffrac- tion grating, which then seeds the other arm of the beam as they pass together through the amplifier crystals.
(b) Photo of the OPO in operation at 550 nm. This is achieved by setting a phase matching angle of∼32◦between the crystal
axis and pump beam, so that, by conserva- tion of momentum, the 355 nm photons split into a 550 nm signal beam and a 999 nm idler beam.
Figure 3.11: The Sunlite EX OPO laser used in this work, which acts as a coherent tune-able photon source with narrow bandwidth.
26 27 28 29 30 31 32 33 34
Phase matching angle (degrees) 400 600 800 1000 1200 1400 1600 1800 Wavelength (nm)
Sunlite EX OPO phase diagram
Signal Idler
Figure 3.12: OPO phase matching diagram, showing the signal and idler frequencies which correspond to a particular angle between the crystal axis and laser pump beam. This allows for the laser frequency to be tuned to anywhere within the range 445-1750 nm.
3.4.2 Direct YAG measurements
While the YAG laser was originally installed to operate as a pump laser for an OPO, modifications were made to the laser connecting chamber to allow for the direct YAG
output to be used in the experiment. This mode of operation has limited wavelength flexibility (1064, 532, 355, and 266 nm) but provides much higher pulse energies (∼300− 2000 mJ) than what can be achieved with the OPO (∼5−50 mJ). From the Beer-Lambert law, the number of photoelectron emitted in a pulse is given by,
jelecφ[1−e−ρσ`], (3.4)
wherejelec is the current of photoelectrons,φis the photon flux,ρis the density of ions,σ the photodetachment cross section, and`the interaction path length. Therefore the num- ber of photodetachment events to occur is directly proportional to the number of incident photons. This makes direct Nd:YAG measurements a very useful mode of operation for target species with low ion counts. It can also be useful for primary measurements of a new target molecule, where a high count rate can quickly reveal the important aspects of the target molecule.
The laser exits the Nd:YAG, where a dichroic mirror deflects the beam through an exit port installed in the chamber connecting the YAG and OPO lasers. A series of prisms then transport the beam from the optics table over towards the ion beam, passing through aλ/2 waveplate and BBO polariser to ensure high purity vertical laser polarisation is achieved at the experiment. The laser beam is then telescoped down to a 2mm spot size before entering the ion beam interaction region through an entry window. A picture of the interaction region is given in Fig. 3.13.
Spatial overlap of the 2mm ion packet and 2mm laser pulse can also prove difficult. Right angle prisms are used to steer the laser beam through the interaction region, while electrostatic x,y,z deflectors on the second potential switch can be used to steer the ion beam. The centre of the interaction region is marked as a line on the optical bench, providing a reference point to aid with the alignment, with vertical alignment achieved using marked rulers.