## 2 Materials and Methods
### Thermally Assisted PE  
The TAPE measurement apparatus was shown in detail previously [5; 13]. We briefly describe the measurement apparatus and its operation procedure. The measurement chamber was a gas-flow-type Geiger counter. Q gas (Takachiho, Tokyo, Japan) consisting of a mixture of 99% He gas (purity of 99.999%) and 1% _iso_-C\({}_{4}\)H\({}_{10}\) (isobutane) vapor was used as the counter gas. The electron counting system consisted of a radiation counter and a linear count rate meter (Ohyo Koken Kogyo, Tokyo). The light irradiation was performed using a system consisting of a grating monochromator and a wavelength drive unit connected to an ultraviolet (UV) source ([PERSON], Niiza, Japan). The UV source was a deuterium lamp (Hamamatsu Photonics). The heating system consisted of a temperature controller equipped with a chromel-alumel thermocouple ([PERSON], Akishima, Tokyo).  
Firstly, a sample was mounted on the sample holder of the measuring chamber, and then the Q gas was continuously flowed into the chamber at atmospheric pressure at a rate of 100 sccm, where the sample surface was directly exposed to the Q gas. The sample was placed 20 mm below the grid of the cathode, and the area of the sample exposed to the incident light was \(0.5\times 3\) mm\({}^{2}\) in the center of the sample. In all experiments, the direction of the light irradiation was approximately normal to the plane of the sample.  
The intensity of TAPE was measured in two experimental methods: (1) during a temperature scan under light irradiation at a fixed wavelength, and (2) during a wavelength scan at a fixed temperature. The PE results from the scratched samples during the temperature scan were reported previously [5; 6]. Here, we briefly describe the operation conditions used in the temperature scan. Under 210-nm-wavelength light irradiation, in the first cycle, the temperature of the sample was increased from 25 to 339 \({}^{\circ}\)C (Up1 scan) (time required for the scan = 950 s) and then decreased from 339 to 40 \({}^{\circ}\)C (Down1); subsequently, a second cycle of an increase from 40 to 339 \({}^{\circ}\)C (Up2) and a decrease from 339 to 25 \({}^{\circ}\)C (Down2) was performed. In the Up1 and Up2 scans, the sample was heated at the rate of 20 \({}^{\circ}\)C/min. The power of the 210-nm-wavelength light irradiation (photon energy = 5.904 eV) was 110 nW (photon flux = \(1.16\times 10^{11}\) photons/s). This power was used to continuously irradiate the sample area during the temperature scan  
Next, the PE measurement during the wavelength scan was carried out over the wavelength range 300-200 nm at the scan rate of 0.4 nm/s (time required for the scan = 250 s) at three temperatures of 25, 200, and 339 \({}^{\circ}\)C. The wavelength scans at the temperatures of 200 and 339 \({}^{\circ}\)C were performed after the PE measurement was conducted by increasing the temperature to these final temperatures in the same manner as in the Up1 scan under 210-nm-wavelength light irradiation mentioned above. Here, it is noted that the sample scanned at 25 \({}^{\circ}\)C was not previously exposed to the 210-nm incident light. A newly scratched sample was used for a wavelength scan at each temperature of 25, 200, and 339 \({}^{\circ}\)C, and the wavelength scan was conducted twice at each temperature. The dependence of the power of the UV source on the incident light wavelength was measured. The power during the wavelength scan was in the range of 93-110 nW for the wavelength range of 200 nm (6.20 eV) to 240 nm (5.17 eV). The power then slowly decreased with increasing wavelength from 245 to 300 nm, reaching 20 nW at 300 nm. In Figure 2 in Reference [7], the plots of (electron emission intensity)\({}^{1/2}\) against the photon energy are shown, giving an approximately straight line in the incident photon energy range of 6.1-5.17 eV. To estimate the value of \(\alpha A\), the average photon flux, \(1.13\times 10^{11}\) photons/s, was obtained using the power of incident light in the range 6.20-5.17 eV (200-240 nm) [5].