In this thesis, OH and OD electronic transition frequencies have been determined using excitation with a narrow-linewidth continuous wave (CW) laser. Although, each measure-ment is Doppler broadened with a linewidth on the order of 8 MHz, the uncertainty of the fitted line positions is at the 10 kHz level. This is similar to the precision achieved on Doppler broadened molecular transitions in molecular oxygen (O2) by Bielska et al.[213]. A fit of an effective Hamiltonian model to the measured line positions supplies a set of refined A-state spectroscopic constants and residuals to the applied model. Compared to previous works, the uncertainty of the parameters has been reduced (Section 10.5). In particu-lar, the band origin and the rotational constant are determined with significantly higher precision. The residuals of the effective Hamiltonian fit are an indicator of the accuracy of the individual measured transitions frequencies. The deviations between the observed and the calculated transition frequencies are, in general, within 100 kHz (Section 10.3).
This rather small deviation confirms our trust in the measured transition frequencies and the provided spectroscopic constants, even if previous measurements suggest different parameter values.
10.6. Conclusion
Chapter 11
Outlook and Summary
This thesis has presented a precision laser system for spectroscopy on small molecules, able to measure and stabilize a narrow linewidth continuous wave (CW) laser in the ultraviolet (UV) as well the infrared (IR). A measurement series ofA2Σ+, v0 = 0←X2Π3/2, v00= 0, J00 = 3/2 transitions in the hydroxyl radical (OH) and the deuterated hydroxyl radical (OD) served as a benchmark system of this new apparatus. Potential line shifting effects have been considered, for instance, the Zeeman effect, the saturation, the AC-Stark shift, and the retroreflection quality. By accounting for these effects, we have been able to determine the zero-field transition frequencies with an uncertainty of less than 100 kHz.
The zero-field transition frequencies, reported in this thesis, have been combined with previous data[49,207,209,210], and used to refine the effective Hamiltonian parameters of the A, v0 = 0 state in OH and OD. In particular, the band origin and the rotational constant have been determined with several orders of magnitude higher precision.
11.1 Increasing the Performance of the Setup
The magnetic field and the corresponding Zeeman effect has a significant influence on the measured line positions on the OD measurements. In retrospect, we underestimated the effect the 75µT ambient magnetic field in the laboratory would have on the measured spectra. This oversight complicated the analysis of the spectra and probably reduced the available precision of the measured transition frequencies. The relatively high laser power needed to overcome the low signal to noise ratio (SNR) and counter-propagating laser beams used to correct for Doppler shifts could have resulted in saturation dips and improved the resolution of the measurements. Thus, the two counterpropagating laser beams in the Doppler reduces measurement setup saturate the transition. Unfortunately, the line splitting inside the magnetic field due to the Zeeman effect leads to multiple saturation dips, which are not at the center of the transition, but instead distort the measured spectrum. Since then, the experimental apparatus has been modified so that future measurements can be performed at zero magnetic field. In particular, two Helmholtz
11.1. Increasing the Performance of the Setup coils with a diameter of 800 mm along the axis of the laser beam (14µT) and two additional coils with a diameter of 400 mm along the molecular beam (75µT) have already been attached to the vacuum chamber. A translatable and rotatable Hall sensor makes it possible to detect magnetic fields along both axes at the molecule interaction region with the laser. Based on the measured field strength, the current inside the Helmholtz coils can be tuned to null-out the ambient magnetic field. The magnetic field along the third axis is neglected since it is at the order of magnitude of the measurement uncertainty (2µT).
The laser power leads in some measurements to saturation effects, although the precise location of the saturation dips often stays hidden inside the noise floor. An increase of the SNR would open the possibility of Doppler-free saturation spectroscopy. After the measurements, we identified multiple sources of avoidable noise. The major contribution to the background noise is the dissociation pulse of the excimer laser. Although color filters block the laser wavelength in front of the photomultiplier tube (PMT), the fluorescence light generated inside the quartz tube at the nozzle of the molecular beam goes through the filters. This fluorescence also persists for the few milliseconds the molecules require to travel from the source to the laser interaction region. Thus, dissociating the molecules without the quartz tube would get rid of this source of background. Another source of fluorescence light was the fused silica windows of the vacuum chamber for the spectroscopy laser, which have since been replaced by calcium fluoride (CaF2) windows, which have no detectable fluorescence. The last potential noise source is scattered light from the light baffles around the spectroscopy laser. Although the beam diameter at the center of the interaction region is around 1 mm and the diameter of the light baffles is 5 mm, some minor fraction of the beam scatters into the PMT. Thus, we increased the diameter of light baffles to 10 mm and simultaneously cleaned the mode profile of the laser beam using a single mode fiber. Additionally, the fiber connects the laser setup on the floating laser table with the stationary vacuum chamber, which makes continuous adjustments of the retroreflection obsolete. These changes will increase the SNR significantly.
The data processing is based on an analog signal from the PMT, recorded on a digital oscilloscope. The experiment was performed with a repetition rate of 10 Hz, partly because the oscilloscope could not be read out much faster than this and did not have an appropriate averaging function built in. The pressure inside the vacuum chamber, which increases with the repetition rate of the valve, could have also become a limiting factor, though later tests proved a reliable valve operation up to 50 Hz, with additional cooling to protect the valve from overheating. Higher repetition rates are only possible by producing narrower molecular pulses, which is not feasible with the current valve. However, an in-house developed, corrosive resistant piezoelectric transducer (PZT) based valve has been shown to generate shorter pulses. The corresponding adapter for the vacuum chamber is ready
Chapter 11. Outlook and Summary
to use, but the valve itself has not been tested yet with nitric acid (HNO3). This valve potentially allows experimental repetition rates up to 250 Hz, which is the maximum repetition rate. However, the faster repetition rate produces more data, and the oscilloscope becomes insufficient. The measurement speed, as well as the sensitivity, can be improved by counting the electrical pulses generated by individual photons reaching the PMT. These pulses are small, short in duration, and highly variable but can be converted to transistor-transistor logic (TTL) compatible pulses with constant width using a discriminator. Thus, the data acquisition needs to change. In particular, the oscilloscope has been replaced with a discriminator, which generates a sequence of identical pulses based on the signal from the PMT. Afterwards, the pulse train is using a low-cost logic analyzer which can continuously record a digital signal at 24 mega samples per second (MSPS). It is expected that future measurements will be at least collected five times faster than the data in this thesis. In case of a successful operation of the in-house valve, maybe even 25 times faster.
To conclude the improvements on the measurement setup, we expect an increase of the transition line precision of at least one order of magnitude. But instead of measuring the same transitions again with a higher precision, it is prudent to move on to a different system.