Field Ionization of C2H5I in Supercritical Argon near the Critical Point

Field ionization of C2H5I doped into argon is presented as a function of argon number density along the argon critical isotherm. These data exhibit a decrease in the argon induced shift of the C2H5I ionization energy near the critical point similar to recent field ionization measurements of CH3I/Ar. We show that this decrease is due to the interaction between argon and the quasi-free electron arising from dopant field ionization and is, therefore, independent of the dopant. The energy of the quasi-free electron is calculated in a local Wigner-Seitz model containing no adjustable parameters to within ±0.2% of experiment. PACS numbers: 33.15.Ry, 34.30.+h, 31.70.-f, 31.70.Dk Field ionization of molecules doped into perturber fluids provides a means of probing fluid structure as a function of perturber number density. We have recently reported a decrease in the perturber induced shift of the CH3I ionization energy near the critical point of argon [1]. This decrease contrasts sharply with the increase observed in the density dependent solvatochromic shift of vibrational and UV-visible absorption bands reported by numerous groups [2] in various perturbers. The difference in behavior stems from the nature of the dopant/perturber interactions in the two cases: The density dependent energy shift of vibrational and UV-visible absorption bands is primarily sensitive to the local density and polarizability of the perturbing medium, whereas the density dependent shift of the dopant ionization energy ∆D(ρP) in dense media can be written as a sum of contributions ∆D(ρP) = V0(ρP) + P+(ρP). (1) In this expression, P+(ρP) is the shift due to the average polarization of the perturber by the ionic core, V0(ρP) is the quasi-free electron energy in the perturbing medium, and ρP is the perturber number density. In modeling the CH3I ionization energy near the critical point of argon [1], we showed that while P+(ρP) shifted in a manner similar to that observed for vibrational and UV-visible bands [2], V0(ρP) did not. Nevertheless, V0(ρP) could be accurately predicted within a dopant-independent local Wigner-Seitz model at noncritical temperatures [1] and along the critical isotherm [3]. Recent theoretical studies on vibrational and UV-visible bands [4, 5, 6, 7], however, have Letter to the Editor 2 shown that the dopant can induce larger density fluctuations in the perturbing medium near the critical point in comparison to density fluctuations in the neat perturber. Since V0(ρP) is dependent upon the local perturber number density [1, 3], this change in local density might lead to a slight dopant dependence in V0(ρP). In this Letter, we report the field ionization of C2H5I high-n Rydberg states in supercritical argon along the critical isotherm near the critical density. These data also show a decrease in the perturber induced shift of the dopant ionization energy near the critical point. The differences between the C2H5I results and the previously reported CH3I/Ar [3] results are shown to arise solely from the average polarization of the perturber by the ionic core (i.e., P+(ρP)) and not from V0(ρP). Therefore, for the case in which the number density of the dopant is much less than that of the perturber, dopant effects on the perturber local density fluctuations play no role in the mechanism leading to the density dependence of V0(ρP) around the critical point of the perturber. The local Wigner-Seitz model [1, 3] employed in CH3I/Ar is used here to predict V0(ρP) and ∆D(ρP) for the C2H5I/Ar system to within ±0.2% of experiment. C2H5I (Sigma, 99.1%) and argon (Matheson Gas Products, 99.9999%) were used without further purification. The absence of trace impurities in the spectral range of interest was verified by the measurement of low density absorption spectra of C2H5I, and of both low density and high density absorption spectra of argon. Both the gas handling system and the procedures employed to ensure homogeneous mixing of the dopant and perturber have been described previously [3, 8]. Prior to the introduction of C2H5I, the experimental cell and gas handling system were baked to a base pressure of 10−8 Torr, and in order to ensure no perturber contamination by the dopant (which was present at a concentration of < 10 ppm), the gas handling system was allowed to return to the low 10−7 Torr range before the addition of argon. Field ionization measurements were preformed using monochromatized synchrotron radiation [8, 9] having a resolution of 0.9 Å (8 meV in the spectral region of interest). The copper experimental cell, capable of withstanding pressures of up to 100 bar, is equipped with entrance and exit MgF2 windows (1 cm pathlength) and a pair of parallel plate electrodes (stainless steel, 3 mm spacing) oriented perpendicular to the windows [3, 8, 9]. This experimental cell is attached to an open flow cryostat and resistive heater that allowed the temperature to be controlled to within ±0.2◦C. In order to prevent liquid formation in the cell during temperature stabilization, the set point for the critical isotherm was chosen to be −121.5◦C, near the argon critical temperature of −122.3◦C. The intensity of the synchrotron radiation exiting the monochromator was monitored by measuring the current across a Ni mesh intercepting the beam prior to the experimental cell. All photoionization measurements were normalized to this current. Field ionization spectra were also energy corrected for the effects of both the low field FL and high field FH (used to generate the field ionization measurement [9]) by I0(ρP) = IF (ρP) + cD(F 1/2 L + F 1/2 H ), where IF (ρP) is the zerofield dopant ionization energy, IF (ρP) is the dopant ionization energy perturbed by the electric field, and cD = 3.0 ± 0.5 × 10−4 cm V−1/2 for C2H5I [3]. For all field ionization measurements reported here, FL = 1667 V/cm and FH = 8333 V/cm. Figure 1a presents the argon induced shift of the C2H5I ionization energy ∆EtI(ρAr) near the critical isotherm of argon, in comparison to that for noncritical isotherms [3]. These data show a clear decrease in the density dependent shift of ∆EtI(ρAr) near the argon critical density (ρAr = 8.0 × 10 cm−3) similar to that observed from field ionization measurements of the CH3I/Ar system (cf. figure 1b) [1]. From a careful perusal of figure 1, one can see that the energy shift for the noncritical Letter to the Editor 3 Figure 1. The experimental argon induced shift of (a) the C2H5I ionization energy ∆EtI(ρAr) and (b) the CH3I ionization energy ∆MeI(ρAr) plotted as a function of argon number density ρAr at (¥, N) various noncritical temperatures [3] and near (•, H [1]) the critical temperature of Ar. The solid lines are provided as a visual aid. isotherms is slightly smaller for the C2H5I/Ar system in comparison to the CH3I/Ar system, and that the change in the energy shift due to critical effects is also smaller. In order to determine if these differences arise from V0(ρP), the average ion/perturber polarization energy P+(ρP) must be evaluated and subtracted from ∆D(ρP). For this system, P+(ρP) was calculated using [1, 3]