# Resonant Laser Ionization and Fine-Structure Study of Silver in an Ablation Plume
## 4 Experimental Conditions and Results
### Effect of Laser Linewidth on Doppler Broadening  
For the next part of the experiment, an effort was made to study the Doppler-affected scenario and the effect of the linewidths of the excitation laser lights.  
The experimental condition changes for these scans were: (a) nearly collinear injection of the excitation lasers and, (b) larger beam diameter of the first step laser, increased to 1-1.5 mm. These changes enabled us to work with a larger interaction volume which contained a higher velocity distribution, resulting in Doppler broadening.  
Figure 7 shows a wavelength scan of the first transition step affected by Doppler broadening. As can be seen, the Gaussian contribution increased to 0.861 pm compared to 0.007 pm in the Doppler-free case shown in Figure 5, thereby increasing the linewidth of the resonance profile to \(\sim\)2.7 pm (7.5 GHz) from \(\sim\)1.2 pm (3.3 GHz). This is the frequency range within which Ag\({}^{+}\) ions can be produced using the laser wavelengths with the given linewidths (see Table 2).  
Figure 5: First transition step wavelength scan 4d\({}^{10}\)5s\({}^{2}\)S\({}_{1/2}\)\(\rightarrow\) 4d\({}^{10}\)5p \({}^{2}\)P\({}_{3/2}^{o}\) Scanning range = 28 pm (77.9 GHz).  
Using lasers of narrow linewidths for ion production entails the disadvantage of losing many atoms which are affected by the Doppler effect. To avoid this loss, lasers of large linewidths are used for ion production in laser ion sources (LIS).  
To study this scenario, the pumping power in the dye oscillator of the second dye laser was increased to push the gain of the cavity far beyond the gain threshold, thereby allowing additional modes to resonate effectively in the oscillator cavity. The pumping power was increased to such a level that, not only the extra modes were created in the oscillator cavity, but these modes were power-broadened to form a nearly continuous broad laser profile with an effective linewidth of \(\sim\)10-15 pm, corresponding to \(\sim\)16.9-25.3 GHz. At this stage, the amplification of the laser beam exiting from the oscillator cavity was reduced to maintain similar power to the central mode by carefully monitoring the intensity of the ion signal. This procedure was performed to emphasize the effect of the spectral linewidth of the excitation laser light. The overall laser power of the second dye laser in this configuration was 850-900 uW.  
Figure 8 shows the resonance profile of the first-step wavelength scan with the second-step laser kept in multi-mode. The linewidth of the profile showed a significant increase to \(\sim\)7.5 pm, corresponding to \(\sim\)20.9 GHz. Compared to Figure 7, the area under the curve increased by \(\sim\)3.5 times, indicating proportional change in the number of ions collected. In other words, if the first-step laser had a linewidth of around 20.9 GHz, it would produce a similar amount of ions by just being maintained at the resonant wavelength, 328.163 nm. This represents clear qualitative confirmation that broad linewidth of the excitation lasers can produce higher ion production in a hot environment where the Doppler effect is dominantly present.  
Using lasers of linewidth around 10 GHz is quite common for the purpose of ion production in laser ion sources to capture Doppler-shifted atoms and to ensure higher ion production. However, the use of a much larger linewidth increases the chance of probing other nearby atomic lines of other elements, which could compromise the chemical selectivity of the laser ion source.  
Figure 7: First-step Doppler broadening; scanned with second-step laser operating as single-mode laser @421.402 nm Scanning range = 68 pm (189.3 GHz).