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IR and UV irradiation at three wavelengths to probe reaction dynamics of Cl in solid para-H₂

OR

IR and UV irradiation at 355 nm, 309 nm, and 416 nm of Cl-doped solid para-H₂

1.                Introduction

UV photodissocation of dopant molecules in low-temperature matrixes serve as models to investigate condensed-phase reaction dynamics.^{1, 2} Matrix isolation spectroscopy (MIS) combined with UV photodissocation yields a useful technique to study of reactive species.^{3, 4} These studies provide model systems to understand the spectroscopy and reaction dynamics of these unstable species.

Classical matrix hosts such as rare gas atoms (Ne, Ar, Kr, Xe) and N2 are commonly used in MIS due to their chemical inertness and weak intermolecular perturbations of the system. Rare gas matrices are also rigid matrix hosts which puts significant limits on the study of chemical reactions at cryogenic temperatures. This rigidness restricts the diffusion of the reagents and thus the temperature of the matrix must be raised to induce mobility.^{5, 6}

Solid molecular hydrogen has unique properties that make it a better matrix host for the study of low temperature reactions. Para-hydrogen (p-H₂) mimics rare gas matrixes but has two important distinctions: the zero-point energy in p-H₂ crystals is a fraction of the binding energy of the solid making them quantum crystals⁷ and p-H₂ molecules can be excited from IR absorptions within the solid.⁸

The Cl + H₂  HCl + H reaction is a hydrogen abstraction reaction that has been studied extensively in both the experimental and theoretical regimes.⁹ The barrier for reaction with zero-point corrections is ~1990 cm^-1 and is 360 cm^-1 endothermic.¹⁰ After photodissociation of the Cl₂ molecule there are a few possible reactions listed below

Cl₂ + h  2Cl*                                                                 (1)

Cl* + H₂(=0, J=0)  HCl + H                                                   (2)

Cl* + H₂(=1, J=0)  HCl + H                                                   (3)

Cl + H₂(=1, J=0)  HCl + H                                                    (4)

The nascent or translationally hot Cl atoms are designated Cl* and is produced with high translational energies within the p-H₂ crystal. The photoejected Cl* atom can potentially react with ground state p-H₂, however, if the nascent Cl* atom does not react and equilibrates within the matrix it cannot react with p-H₂ (=0, J=0) because it cannot surpass the endothermicity of the reaction even through tunneling.¹¹ Equilibrated Cl atoms can react with vibrationally excited H₂(=1) molecules via reaction 4 because the H₂ vibrational energy is used to overcome the endothermicity of the reaction.^{3, 11, 12} There is some possibility of side reactions involving the equilibrated Cl atom reacting with the H atoms produced but the initial concentrations of the Cl₂ were intentionally kept low to prevent significant yields via the side reactions.

The experimental work done here was motivated from previous theoretical¹³ and experimental work done by our group studying in situ photodissociation of Cl₂ in solid p-H₂ in order to produce isolated Cl atoms. ^{11, 14} Theoretical work conducted by J. Manz et al. helped predict outcomes of the photodissociation of Cl₂ in p-H₂ crystals using various IR and UV irradiation schemes. They predicted photodissociation of Cl₂ in solid p-H₂ (=0, J=0) would produce isolated Cl atoms which has also been confirmed through our group’s studies. They further predicted that by exposing our crystal to IR light, the Cl + H₂ reaction would be enhanced due to vibrationally excited H₂(=1) molecules which was also confirmed in our studies via reaction 4.¹⁴

The in situ photodissociation of dopant molecules in solid p-H₂ is almost free from the “cage effect” due to para-hydrogen’s large lattice constants as well as large amplitude zero-point vibration.¹⁵ This makes in situ photochemistry a viable method to produce well isolated fragments for further reaction studies. While a reaction of the photofragments with rare gas hosts is rare, in solid molecular hydrogen many chemical species will react with the host. This limits the study of some chemical species but also permits side reactions with the hydrogen host to be used as a strategy to produce H atoms.^{16, 17}

The translational energy of the nascent translationally hot Cl* atom depends on the photolysis wavelength. In this case there are three different wavelengths used while conducting these experiments: 309 nm, 355 nm, and 416 nm. The translational energy of the Cl atom is  defined as

(5)

where E_trans is the translational energy of the photoejected Cl atom, E_ph is the photon energy, and D₀ = 2.4793 eV is the dissociation energy of Cl₂¹⁸. The values of E_trans corresponding to each wavelength are reported in Table 1. E_trans does not take into account the center of mass frame of the Cl atom within the solid crystal. In order to calculate the total energy of the reaction, the following equation is used

(6)

For 309 nm, 355 nm, and 416 nm, the E_rxn is 334 cm^-1, 221 cm^-1, and 109 cm^-1, respectively. As mentioned previous, the endothermicity barrier of reaction is 360 cm^-1. While the Cl atom has high enough translational energy, when the center of mass frame is taken into account the Cl atom does not have enough total energy to surpass the endothermicity and tunnel through the barrier.

The absolute photoabsorption cross section of Cl₂ changes as a function of wavelength as seen in Figure 1. The maximum absorbance of chlorine is approximately at 330 nm with wavelengths of 309 nm and 355 nm having high cross sections of 1.69 x 10^-19 cm²/photon and 1.62 x 10^-19 cm²/photon, respectively. At 416 nm, the cross section drops down to 1.16 x 10^-20 cm²/photon.¹⁹

2.                Experimental
   1.              Crystal preparation

The methods^{20, 21} used to prepare Cl₂-doped p-H₂ crystals have been reported previously,^{11, 14, 22} and here, we will focus on the specifics of the IR + UV photochemical measurements. In brief, the crystals are synthesized by co-depositing gas flows of Cl₂ and precooled p-H₂ (99.99%) gas onto a BaF₂ substrate maintained at ∼2.5 K with a sample-in-vacuum liquid helium bath cryostat. The samples were not thermally annealed and exhibit a crystal morphology consisting of mixed hexagonal close-packed and face-centered cubic structures.²³ The gas flow rates are adjusted to grow approximately 2.5(1) mm thick samples with Cl₂ concentrations of ≤50 ppm. The desired concentration of Cl₂ is around 50 ppm to work within the dilute matrix isolation limit in order to minimize Cl atom recombination during UV photolysis and to minimize the importance of secondary reactions involving the H-atom products.

2.               UV photolysis

In order to study the IR + UV-induced HCl photoproduction kinetics, two different types of experiments were performed at three different wavelengths: 355 nm, 309 nm and 416 nm. The first experiment, designated UV, involved UV photolysis of the sample in the absence of any external IR light source in order to examine the kinetics of reactions 1 and 3. UV photolysis is involved using the 355 nm output of a 10 Hz Nd:YAG laser (Spectra Physics Laboratory-170-10) with approximately 7−8 ns pulse widths. To achieve 309 nm and 416 nm outputs, the Nd:YAG laser utilized Raman shifting with stokes and anti-stokes shifts. The unfocused 8 mm diameter UV laser beam has an output energy of 4(±0.5) mJ per pulse as measured with a power meter just before a CaF₂ photolysis window on the vacuum shroud of the helium cryostat. The UV beam impinges on the crystal at 45° with respect to the surface normal of the BaF₂ substrate. In the UV photolysis experiments, the sample is irradiated in 5 min exposure intervals while maintaining the sample at 1.8 K. After each UV photolysis exposure, the amount of HCl produced is measured using a FTIR spectrometer (Bruker IFS 120 HR) with appropriate filters in the IR beam to prevent HCl production via reaction 4.^{3, 11, 12}  This procedure is repeated for up to 4 h total of UV exposure to fully photodissociate all of the Cl₂ molecules in the sample.

3.              Sequential UV and IR

After the UV photolysis is complete, the sample is exposed to unfiltered IR light from the FTIR spectrometer to chemically transform all of the unreacted Cl atoms into HCl. This allows the HCl signal produced from UV photolysis to be compared to the total HCl signal that would be produced if all of the Cl atoms were to react to form HCl. The latter process is designated IR, and the IR-induced reaction 4 is achieved with the unfiltered cw IR output from the FTIR spectrometer equipped with a CaF₂ beamsplitter and tungsten source with a 4.0 mm aperture setting on the IR source. This combination of beamsplitter and IR source produces broad-band near-IR radiation with peak intensity in the 4000−5000 cm⁻¹ range where solid p-H₂ absorbs. The IR beam is focused with 90° off-axis parabolic mirrors to a nominal spot size of around 1−2 mm diameter within the UV-irradiated part of the crystal. The sample is IR irradiated for time periods ranging from 1 to 60 min for a total IR exposure time of up to 4 h. This experiment allows us to study the UV- and IR-induced HCl production kinetics separately.

4.              Simultaneous UV + IR

The second type of experiment performed is designated IR + UV and consists of simultaneous exposure of the Cl₂-doped p-H₂ sample to both the IR and UV sources for specific time intervals ranging from 5 s to 1 h, for up to ∼3 h of total IR + UV exposure. Once again, the sample is maintained at 1.8 K, and after each IR + UV exposure, the amount of HCl produced is measured using the FTIR with appropriate filters in the IR beam. The energy of the UV laser is adjusted to 4(±0.5) mJ per pulse to match the conditions of the UV photolysis experiments, and the IR light is generated using the FTIR with a CaF₂ beamsplitter and unfiltered tungsten source with a 12.5 mm aperture setting. The IR + UV experiments allow for the investigation of the amount of HCl produced by consecutive reactions 1 and 3.

The HCl concentration is monitored during both UV and IR + UV experiments using the FTIR spectrometer equipped with a tungsten source, a CaF₂ beamsplitter, and a liquid-nitrogen-cooled InSb detector. Some experiments use a 3861cm^-1 long-pass filter (LPF) in the IR beam to record spectra from 1900-3900cm^-1. Other experiments do not use a LPF and collect spectra from 1900-10000cm^-1. All spectra are averaged over 36 scans with 0.05cm^-1 resolution. The IR beam is propagated through the BaF₂ deposition substrate and sample with the main optical axis parallel to the substrate surface normal (0° incidence). The entire optical path outside of the FTIR and cryostat vacuum shroud is purged with dry N₂ gas to reduce atmospheric absorptions.²²

3.                Results and discussion
   1.              UV only

There are two different types of experiments conducted in this study. The first type is sequential IR and UV irradiation which will be discussed here, and more specifically the UV irradiation of the solid Cl-doped pH₂ crystal. To monitor the H + Cl₂ reaction, the HCl R₁(0) rovibrational transition is monitored with respect to time of UV photolysis. At 4K, only the j=0 ground rotational state is populated so a single peak is exhibited and used to measure the concentration of HCl.

The HCl R₁(0) peak is located at 2892-2894 cm^-1 where this range includes both isotopes of chlorine, ³⁵Cl and ³⁷Cl. As seen in Figure?, (b) shows the HCl R₁(0) peak growing with increasing UV photolysis time. Clusters A, B, and C, as well as (HCl)₂ also continue to grow with UV photolysis time. The Cl spin-orbit excited atom at around 945 cm^-1 in Figure? (a) grows with UV photolysis exposure. Cluster A is located at around 2877 cm^-1, Cluster B around 2873 cm^-1, and Cluster C at 2865 cm^-1. (HCl)₂ appears around 2834 cm^-1.

When comparing the integrated absorbances of the cluster peaks, the dimer, and the R₁(0) peak, the dimer peaks are about 15% of the R₁(0) peak under UV-only conditions. The cluster peaks are about 5% of the R₁(0) peak. The absorption strength of the dimer and cluster features has a relative IR intensity that is 2.5 times stronger than the R₁(0) tells us that the HCl R₁(0) peak is isolated and a good measure of the relative concentration of HCl in the pH₂ crystal when that peak’s absorption is integrated.²⁴

In the UV-only regime, Cl* atoms are seen to slightly react due to the small increase in integrated absorption HCl prior to IR irradiation. This is contradictory to the calculated E_rxn for each wavelength.

The values from Table1 are E_rxn= 221.4 cm^-1, 334.8 cm^-1, 109.8 cm^-1, respectively for wavelength 355 nm, 309 nm and 416 nm. As seen in Figure 1, the E_rxn for each wavelength are all below the threshold of 1500 cm^-1 to form Cl + H₂  HCl + H as well as the endothermicity of the reaction requiring 360 cm^-1 for quantum tunneling to surpass the barrier.¹⁰

However, the “target” p-H₂ is embedded in the quantum crystal which means the Cl atom does not collide with one single H₂, rather a Cl atom collides with an H₂ molecule that is in contact with other H₂ molecules. Because of the contact with other nearest neighbors in the crystal, the maximum collision energy within the solid is slightly greater in a head-on collision. With this increase in energy, it still isn’t possible to surpass the 1500 cm^-1 barrier to reaction. Based on this reasoning, photolysis of the Cl-doped pH₂ crystal results in isolated Cl atoms.

2.               UV then IR sequentially

IR light using a Tungsten source, irradiates the solid pH₂ crystal to allow the Cl atom that has reach thermal equilibrium to react with vibrationally excited H₂ molecules to form HCl via reaction 4. As seen in previous studies, the HCl R₁(0) grows significantly once the solid has been exposed to IR light.¹⁴

As seen in Figure ?, there is still some HCl produced during the UV only regime without vibrationally exciting the pH₂ molecules through IR irradiation. The fraction of HCl produced via reaction 2 during UV only photolysis can be calculated using

PRUV= aUVy0IR+aIR      (7)

where

aUVis the maxium integrated absorbance of R₁(0) for UV photolysis and

y0IR+aIRis the maximum integrated absorbance after sequential UV and IR irradiation.

The probability of the reaction increases under the 416 nm UV photolysis conditions and for all three experiments, the probability of reaction is not zero during the UV only regime as seen in Table ?. In the theoretical study done by Manz et al. and they also found the probability to be nonzero,

PRUV=1.6 x 10-5. Manz hypothesized that the zero-point energy is responsible for the non-zero

PRUV. Heisenberg’s uncertainty principle can be applied to the zero-point motion of the H₂ molecule. As the uncertainty of the position of the H₂ molecules increases due to translational zero-point motion, the reaction threshold also decreases and gives a greater reaction probability.

As seen in Table?, the probability for reaction increases for experiments conducted under 416 nm photolysis conditions. One possible explanation is the increased concentration of spin-orbit excited Cl atoms produced from 416 nm.(citation) Based on experiments conducted by Alexander et al, only at collision energies below the reaction barrier of Cl + H₂  HCl + H will nascent Cl* contribute significantly to the production of HCl and H atoms^{25, 26} via reaction 2. This is due to the available spin-orbit energy equaling ~875 cm^-1 which allow tunneling reactions to proceed by surpassing the endothermicity of 360 cm^-1 to reaction.

Small probability values may also be due to side reactions that are not outlined in reactions 1-4. Another possibility is chlorine recombination, in cases where there is a higher concentration of Cl₂ there is a higher chance of chlorine recombination. This reaction releases about 20000 cm^-1 of energy which could transfer through the solid allowing more reactions to occur than previously predicted.

The different IR and UV components of the kinetics are fit to a first order rate equation except for experiments where the IR light was > 5,200cm^-1. In those cases, the IR portion of the kinetics is fit to a bi-exponential kinetic equation. This is because…?

3.              UV + IR simultaneously

4.                Conclusion

5.                Acknowledgements

6.                References

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