Neutralization of solvated protons and formation of noble-gas hydride molecules: matrix-isolation indications of tunneling mechanisms?
Summary (2 min read)
Introduction
- An intriguing question concerns the experimentally observed decay of the cations at low temperatures, which is not fully understood to date.
- For the low-temperature formation of HArF molecules, the hydrogen atom tunneling is a probable mechanism.
A. Experimental details
- The HF/Ar and HF/Kr matrices 100 m thick were deposited onto a cold CsI substrate by passing Ar and Kr gas AGA over a HF-pyridine polymer Fluka at room temperature.
- In order to prepare deuterated samples, the HF/Ng mixture was passed through a line with deuterated sulfuric acid, and the deuteration degree was up to 90%.11.
- The photolysis of HF was performed with a Kr continuum lamp Opthos emitting in the 127–160 nm spectral interval, the decomposition proportion being typically 20% after 30–60 min of irradiation, limited probably by photogenerated light absorbers.
- Most of the HF/Ng matrices were quite monomeric with respect to HF, and the estimated HF/Ng ratio was 1/2000.
- In particular, the doublet band of HF monomer at 3962.5 and 3953.8 cm−1 dominated in the IR absorption spectra in solid Ar, in agreement with the literature data.
B. Experimental results
- The IR absorption spectra in Fig. 1 demonstrate various steps of an experiment with HF in solid Ar. Some amount of HArF is seen already after the vacuum ultraviolet VUV photolysis and, simultaneously, ArHAr + ions are formed, evidenced by the IR absorption band 3 at 903 cm −1.
- Figure 2 presents the ArHAr + concentration versus the annealing temperature for four experiments with similar annealing periods.
- Redistribution subject to AIP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp siderably as compared with the lowest working temperature see below for experiments at 8 K .
- The present data show that the vibrational excitation of ArHAr + by globar radiation does not accelerate the decay process.
B. Low-temperature formation of HArF
- Now, the authors discuss the low-temperature formation of HArF.
- Indeed, upon electron transfer from F− to ArHAr + the neutralization reaction can locally introduce some energy to a lattice and hence promote the H+Ar+F reaction, and this mechanism may be operating to some extent.
- Furthermore, the H/D isotope effect is quantitatively different for the ArHAr + decay and the HArF formation.
- The short separation distance is needed for efficient H atom tunneling.
- Redistribution subject to AIP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp.
IV. CONCLUDING REMARKS
- The authors have studied experimentally the kinetic behavior of various neutral and charged species in UV-photolyzed noblegas matrices doped with hydrogen halides and discussed the involved formation and decomposition mechanisms.
- The authors suggest that this decay phenomenon can occur via a neutralization of the solvated proton by electrons.
- 8,9 Secondly, the authors have found experimentally that the formation of HArF molecules occurs slowly but efficiently in photolyzed HF/Ar matrices already at 8 K.
- The tunneling mechanism is supported by the strong H/D isotope effect.
- There is definitely a room for further experimental and theoretical research of these processes.
ACKNOWLEDGMENTS
- The Academy of Finland supported this work.
- Discussions with Vladimir Feldman are gratefully acknowledged.
- Vladimir Bondybey is thanked for reading the manuscript and for useful comments.
- Redistribution subject to AIP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp.
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Frequently Asked Questions (16)
Q2. How many pulses of HBr were used to photolyze?
The 193-nm radiation of an excimer laser MSX-250, MPB, pulse energy density 10 mJ cm−2 was used to photolyze HBr molecules, and typically 90% of HBr was decomposed after 1000 pulses.
Q3. What is the effect of tunneling on the decay of cations?
The decomposition of the cations can be caused by their reactions with electrons stored in the matrix as a result of photolysis, and the transfer of the electrons to the positive center occurs via tunneling.
Q4. What is the reason for the bleaching of the S2+ photoluminescence?
The direct neutralization of negative charges with concomitant attachment of the electron to the matrixisolated S2+ ions upon irradiation at 266 nm was suggested as the reason for the bleaching of the S2 + photoluminescence.36
Q5. What is the mechanism of the YHY anions?
They can be formed due to the tunneling of an electron from the Y− center to the neutral Y–HY center originated from the photolysis of HY dimers.
Q6. What is the origin of the 1992-cm1 band?
The 1992-cm−1 band probably originates from the hindered rotation of HArF in an Ar matrix, in analogy with some other HNgY molecules.
Q7. What is the reason for the delay in the formation of the HNgY molecules?
It should be stressed that the delayed formation of the HNgY molecules has been usually promoted by annealing activating diffusion of hydrogen atoms in the matrix, with an important exclusion of IR-decomposed HXeI.
Q8. What are the proposed mechanisms for the global mobility of protons solvated in noble-gas?
The tunneling and light-induced mechanisms for the global mobility of protons solvated in noble-gas solids were proposed by Beyer et al.8,9
Q9. What is the effect of the tunneling of electrons on the YHY an?
As seen in the experiments, the YHY − formation is slower at low temperatures than the decay of the cations, which can be caused by larger distances involved or/and a small concentration of Y–HY centers after photolysis.
Q10. What is the way to explain the tunneling mechanism?
The proposed electron tunneling mechanism should be considered as an alternative to the previously suggested mechanisms based on tunneling-supported or light-induced diffusion of protons in noble-gas lattices.
Q11. Why is the NgHNg ++e neutralization reaction strongly exothermic?
The energetics of this system allows the electron-tunneling process because the NgHNg ++e− neutralization reaction is strongly exothermic due to the large ionization energy of hydrogen 13.5 eV .
Q12. How does the ArHAr + concentration decrease after annealing?
25 Upon annealing at 20 and 33 K the ArHAr + concentration decreases; however, even after the 33-K annealing the cations are very visible in the spectra.
Q13. What is the effect of the light neutralization on the H atoms?
In this model, the mobilization of H atoms resulted from sudden light-induced neutralization of the NgHNg + ion, giving kinetic energy for the light H particle.
Q14. How is the electron tunneling process based on the properties of solidstate photolysis?
It follows from the properties of solidstate photolysis that the negative and positive centers are formed in the vicinity of each other, and the distance is comparable with the lattice parameter 0.5 nm .19
Q15. How can the neutralization reaction promote the formation of HArF?
upon electron transfer from F− to ArHAr + the neutralization reaction can locally introduce some energy to a lattice and hence promote the H+Ar+F reaction, and this mechanism may be operating to some extent.
Q16. What is the effect of the globar radiation on proton decay?
The globar radiation is inefficient in promoting the proton decay so that the blackbody radiation is improbable to do this either.