The previously described gaseous sample delivery system for UED3 consisted of a resistively heated nozzle mounted on the top of the vacuum chamber.9,18 The needle
was assembled using stainless steel tubing culminating in a sharpened tip of chromatography tubing with a 180 µm aperture. Outside the chamber, a cylindrical stainless steel sample reservoir was connected to the nozzle with tubing and fittings. The entire gaseous sample delivery line from the reservoir to the needle tip was electrically heated with thermocouple wire inside the vacuum and with fiberglass heating tape on the outside. The temperature of the line was continuously increased starting from the reservoir and ending at the needle tip to avoid any sample condensation and subsequent clogging of the tip opening. This setup was capable of producing tip temperatures of up to 600 K, which were sufficient to vaporize organic molecules with a low boiling point (e.g., nitrobenzene described in Section 5.3), but higher boiling point compounds were out of reach for UED analysis.
To improve the performance of the sample inlet system, we implemented a nozzle capable of reaching temperatures of up to 850 K, using an air heated jacket, as shown in Figure 3.12. The sample inlet tube is analogous to the old design, but is itself enclosed inside a bigger tube. Hot air from a heat gun is delivered through a third tube ending right above the needle, which ensures a continuous and uniform temperature gradient with the tip being the point of highest temperature. This nonresistive heating technique also eliminates any possible interference between an electric field and the electron beam. The gas line outside the vacuum was left unaltered, since the fiberglass heating tapes
employed there are capable of achieving temperatures that were sufficient for the purpose of current experiments.
The overall length of the sample delivery line is rather long (~40 cm) and, consequently, the chance sample decomposition increases. Given the high temperatures and long data acquisition time of ~7 days, during which the sample has to be continuously boiled, high demands are imposed on the surface properties of the delivery line. To minimize any unwanted sample-surface reactions, the entire inner surfaces of the tubing and the fittings, including the micron-sized needle orifice, were chemically passivated with a general-purpose inert layer (Silcosteel, Restek Performance Coating). This modification made it possible to study indole (see Section 5.4), which, previously, had polymerized in an uncoated sample vessel and clogged the needle tip.
After delivering the high boiling point sample into the gas phase, the vapor needs to be kept from expanding to the walls of the UED3 scattering chamber for several reasons. First, the sample vapor will coat the UV laser inlet window, get burnt on it, and lower the overall transmission of the laser light, which in turn reduces the time-resolved signal intensity. Second, the sample will enter the diffusion pump, which, operating at ~250○C, is not hot enough to vaporize the sample. Consequently the diffusion pump would have to be disassembled and cleaned after every experiment. Third, the phosphor screen of the detector would be damaged by the continued deposition of a chemical that has a very low vapor pressure at room temperature and cannot easily be pumped out of the chamber. To capture the sample, a cryo-trap was designed and mounted inside the vacuum directly below the needle orifice, as shown in Figure 3.13a. The whole cryo-trap assembly is held in place by its own liquid nitrogen delivery lines to ensure that nothing
else inside the chamber is in thermal contact with the cryo-trap. The base unit consists of a ring shaped liquid nitrogen reservoir. The actual trap is a thick-walled aluminum cup that is screwed into the top of the reservoir base unit. Liquid nitrogen is continuously delivered into the reservoir unit to ensure uniform temperatures of -196○C. This assembly is capable of trapping about 85% of the total sample delivered. The cryo-trap is covered with a cone-shaped wire mesh (not shown) to avoid charging of the accumulating sample on the cup by stray electron bombardment, which, in turn, will alter the trajectory of the electron beam. Because the mesh is not in thermal contact with the cryo-trap, it does not accumulate much sample itself.
An additional aluminum shield was mounted directly onto the stem of the nozzle. This shield consists of a front shield and a bottom shield, as shown in Figure 3.13b, and satisfies several purposes: (i) The bottom shield covers the cryo-trap from the top and prevents stray electrons from charging the sample. Conversely, it also prevents the electric field of the charged sample to influence the electron beam. (ii) The bottom shield helps to reduce the residual gas pressure above the shield, because the sample vapor escapes through a small central hole and is immediately trapped by the cryo-trap. (iii) The front shield has a pinhole for the electron beam only. Therefore, any stray electrons scattered by residual gas before the interaction region are intercepted/filtered by this grounded shield and never reach the detector. We found that the addition of this shield assembly decreased the contribution of background scattering in our data (see Section 4.1.2) from ~25% to about 10%.