potential on the photocathode to a more positive value than the first dynode.
The spectroscopy laser beam at 308 nm passes a neutral density filter and a 0.8 mm circular aperture approximately 27 cm away from the center of the BBO. Afterwards, the beam propagates approximately 2 m in the air from the laser table to the entrance of the vacuum chamber. Here the beam is recollimated with a fused silica lens (F = 2 m) before it enters the vacuum chamber through an ultrasil fused silica Brewster window.
The 5 mm thick window is directly glued on to the flange (Torr Seal). Inside the chamber, it has to pass one light baffle tube, containing two skimmer like circular apertures of 5 mm diameter. The internal surface of the tube is coated with graphite to shield the PMT from potential stray light. Finally, the beam reaches the interaction region of the OH with an estimated elliptical profile of 0.8×1.0 mm in size. Potential Doppler-shifts can be compensated using a counterpropagating beam along the same axis. Therefore, after the 308 nm beam passes the interaction region and propagates through a second light baffle tube and a window, it is retroreflected on a UV enhanced aluminum coated mirror (R = 93 %). The beam propagates back along the same path and passes the initial 0.8 mm aperture a second time, but now from the opposite direction. The neutral density filter behind the aperture serves this time as a reflector. The intensity of the reflected light on the photodiode is maximized, which corresponds to an optimal superposition of the two counterpropagating beams. The estimated maximum offset between the two beams at the aperture is around 0.1 mm.
9.7 UV Spectroscopy Measurement
Using the precision laser and the molecular beam system, we can now measure the elec-tronic spectra of OH and OD. The 616 nm CW dye laser frequency is monitored with a wavelength meter (Toptica WS7), and a spectrum analyzer displays the optical beat note relative to the nearest OFC mode. The UV wave at 308 nm follows any frequency changes of the dye laser. Therefore, the coarse frequency selective elements inside the dye laser cavity are aligned based on the wavemeter reading. The two cavity PZT mirrors are controlled externally based on the frequency of the optical beat note. The reference signal for the beat note is provided by a computer controlled frequency synthesizer. A change of the reference frequency results in a change of the optical frequency. The dye laser is tuned in steps of 100 kHz, corresponding to 200 kHz in the UV, over the electronic transitions of OH or OD. A PMT detects the fluorescence light resulting from the excitation. The analog signal of the PMT anode is connected to a digital oscilloscope, with a 100 kΩ load to the ground. The signal on the PMT is recorded starting 1 ms after the excimer pulse for a duration of 2.8 ms with a resolution of 2µs. The SNR is improved by averaging around 44 shots at a repetition rate of 10 Hz. The measurement is repeated for each frequency
Chapter 9. Experiment
step, so a complete scan over the transition results in a two dimensional (2D) matrix of the PMT signal as a function of time delay and laser frequency (Figure 9.14a). The
(a) (b)
Figure 9.14:(a) Single scan over an electronic transition in the OH. The plot shows the fluorescence intensity as a function of time delay after the excimer pulse and absolute optical frequency, respectively. The cut-though at single laser frequency displays a single, unaveraged oscilloscope trace with poor SNR. (b) Averaging multiple matrices increases the SNR significantly, allowing a fit to extract the signal amplitude at each frequency step.
frequency steps in each scan correspond to fixed beatnote frequencies
Since the repetition rate of the OFC varies slightly over time, the absolute frequency can be slightly different in each measurement. To correct for this, we continuously record the repetition rate of the OFC on a frequency counter. These measurements are time-stamped and can later be correlated with the data from the oscilloscope. The beat note frequencies in each scan are converted to absolute optical frequencies using
fabs = 2(nfr+fbn), (9.11)
with the averaged repetition rate fr over the time interval of the scan, and n determined using the laser frequency measured by the wavemeter (Figure 9.15a). The prefactor of 2 accounts for SHG of the 616 nm wave into the UV.
Averaging multiple 2D matrices to improve the SNR requires consideration of the changing repetition ratefr between the measurements. The frequency spacing of 200 kHz between each step of the scan is conserved, while the absolute frequency offset in each scan is expressed relative to the first scan (Figure 9.15b). All frequency steps after the first scan are weighted depending on their relative value to the frequencies of the first scan, resulting in a slight frequency shift of all following 2D matrices to match the frequency
9.7. UV Spectroscopy Measurement
Figure 9.15: (a) Frequency counter measurement of the OFC repetition ratefr over the time interval of the first scan (#1). The time scale used is unix time, which counts the number of UTC seconds since the 1st January 1970. (b) The averaged OFC repetition rates of each of the following scans (#2-#12), relative to the averagedfr
of the first scan.
axis of the first scan. For instance, Figure 9.14b results from a weighted average of 12 individual matrices, including the one shown in Figure 9.14a.
Extracting the signal amplitude out of the averaged 2D matrix is the next processing step. The scattered light from the spectroscopy laser at the windows and the light baffles contributes to a constant background. Much more severe at early times is the fluorescence light of the quartz capillary induced by the dissociation pulse. The actual fluorescence light of the molecules peaks at around 1.84 ms after the dissociation pulse, but still within the tail of the quartz capillary fluorescence. Therefore, an independent seven parameter fit at each frequency trace is used to separate the contributions of the background and the signal. In detail, the fitted model consists of an exponential and linear function, which address the background, while a Gaussian function approximates the signal
f(t) = exp[−a(t−t0)] +mt+b
The fit parameters of the Gaussian function describe the amplitude A of the signal, the arrival time of the moleculest1 and the standard deviationσ. The total fluorescence inten-sity of the OH is estimated by subtracting the background contributions and integrating the intensity within a fixed time window ±2σ around t1. This total fluorescence intensity is determined independently for each laser frequency in the averaged matrix. Examples of typical spectra of fluorescence intensity versus laser frequency for OH and OD are de-picted in Figure 9.16a and Figure 9.16b, respectively. The laser induced fluorescence (LIF) measurements start from the ground state X2Π3/2,J00= 3/2 and go to the first electronic excited state A2Σ+. While all hyperfine components are well resolved and separated for OH, which allows a frequency scan over a single transition line. The smaller ground-state
Chapter 9. Experiment
70 80 90
Frequency / MHz - 973 552 700 0.0 Frequency / MHz - 975 734 900 0.0 5/2 transition cluster in the OD with fit.
hyperfine splitting in OD results in transition clusters (Section 2.6.1). The spacing of the transitions is similar to the linewidth which results in blending. Therefore, a scan that covers transitions from all of the closely separated hyperfine ground state levels to the excited hyperfine state becomes necessary. The scanning procedure is repeated for the strongest transitions in the OH and the OD at least twice on different days, which ensures reproducibility of the measurements.