Electric Field Effect on Condensed-Phase Molecular Systems. VI. Field

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Electric Field Effect on Condensed-Phase Molecular Systems. VI. Field-Driven Orientation of Hydrogen Chloride in an Argon Matrix Hani Kang, Youngwook Park, Zee Hwan Kim, and Heon Kang J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b11740 • Publication Date (Web): 04 Mar 2018 Downloaded from http://pubs.acs.org on March 6, 2018

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The Journal of Physical Chemistry

Journal of Physical Chemistry A, 2nd Revised Version

Electric Field Effect on Condensed-Phase Molecular Systems. VI. Field-Driven Orientation of Hydrogen Chloride in an Argon Matrix

Hani Kang, Youngwook Park, Zee Hwan Kim,* and Heon Kang* Department of Chemistry, Seoul National University, 1 Gwanak-ro, Seoul 08826, Republic of Korea

ABSTRACT The orientation state of hydrogen chloride (HCl) molecules in a solid argon matrix was reversibly controlled by applying an external electric field of up to 4×108 Vm–1 using the ice film capacitor method. The rovibrational transitions of the field-oriented HCl were measured by reflection absorption infrared spectroscopy with p-polarized light. Upon application of the external field, free rotation of HCl inside the matrix gradually changed to perturbed rotation and then to a pendular state harmonically bound in the Stark potential well. Further increase in the field strength increased the degree of dipole alignment along the field direction, approaching an asymptotically perfect orientation of the molecules 8

–1

with an average tilt angle of 99%) gases were used as received in commercial gas cylinders. H2O was purified by freeze-pump-thaw cycles. The thicknesses of the constituent molecular films in the sample were estimated from temperatureprogrammed desorption (TPD) measurements. The thickness of the H2O film was determined by comparing its TPD peak area with that of the H2O monolayer formed on Pt(111). The TPD peak area of the Ar film was compared with a H2O film of known thickness by considering the different ionization cross-sections of the gases in mass spectrometric detection.

26, 27

The thickness of 1 ML of amorphous

2

ACS Paragon Plus Environment

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solid water is 5.5 Å and that of solid Ar is 5.7 Å, as estimated from the densities of these molecular films at 7 K.28-30 20, 21

An electric field was applied across the HCl@Ar sample by using the ice film capacitor method. +

An alkali metal ion gun (Kimball physics) was used to deposit Cs ions at a low incidence energy uniformly on the surface of the H2O layer of the ice film capacitor. The voltage developed across the ice film capacitor was measured by a Kelvin work-function probe. The field strength across the HCl@Ar film was estimated using eq 1. =

(1)

where is the capacitor voltage, as measured by a Kelvin probe, and and are the corresponding film thicknesses. The parameters and are the relative permittivities of each film and have the values of 1.6 and 2.0,

31, 32

respectively. The external field strength calculated by eq

1 can be converted to the local field strength by including the contributions of the cavity and reaction fields, according to the classical electrostatic model and Onsager reaction field theory.21, 21

conversion procedure has been described in detail previously.

33

This

It leads to a local field correction

factor of 1.16 for HCl inside a spherical cavity of the Ar matrix, as calculated from reported physical parameters.24, 32, 34 RAIRS measurements were performed at a grazing angle of 84° using a FTIR spectrometer equipped with a mercury cadmium telluride detector. The IR beam was p-polarized using a wire grid polarizer placed in the incident beam path and the RAIR spectra were scanned 2304 times at a –1

spectral resolution of 1 cm .

III. RESULTS AND DISCUSSION We constructed an ice film capacitor device containing a HCl@Ar sample as shown in the inset of Figure 1. First, an Ar film was laid on a Pt(111) substrate inside a vacuum chamber with a thickness of 45±2 monolayers (ML) as the spacer layer. Then, a HCl@Ar film of thickness ranging from 90±3 to 450±16 ML was grown by co-deposition of HCl and Ar gases at partial pressures of a predetermined ratio using separate dosing facilities. Another Ar spacer layer (45±2 ML thickness) was placed on top of the HCl@Ar film. Finally, the sample was capped with an amorphous H2O film having a thickness of 27±1 ML. The temperature of the Pt substrate was maintained at ~7 K during sample preparation. The film thicknesses and the Ar:HCl molar ratio in the HCl@Ar matrix were determined by performing thermal desorption mass spectrometry experiments. The reflection absorption infrared (RAIR) spectra of the HCl@Ar samples are shown in Figure 1. The spectrum in Figure 1(a) shows three major peaks at 2888, 2818, and 2787 cm–1, which are assigned to the HCl monomer, dimer, and trimer in the Ar 3



matrix, respectively, according to the previous IR spectroscopic studies of HCl in noble gas matrices.

35

The broad absorption feature at 2730–2780 cm–1 is most likely due to larger clusters of HCl.35 When the HCl concentration in the matrix was diluted, the intensities of the multimer peaks (2818, 2787, and 2730–2780 cm–1) were decreased relative to that of the monomer peak, as expected from its multimer origin. In the spectrum shown in Figure 1(b), where the Ar:HCl ratio was 870:1, the monomer peak was the strongest and well isolated from the multimer signals. Furthermore, the dilution of HCl concentration sharpened the monomer peak. This change can be attributed to the reduced Stark broadening of the peak at a lower HCl density because the dipole-dipole interaction between the neighbor HCl molecules in the matrix became weaker under these conditions. In the discussions hereafter, we have focused on the HCl monomer peak and its changing behavior under the influence of an applied field, neglecting the multimer features in the IR spectrum.

(a) Ar:HCl = 340:1 (HCl)2 (HCl)3

HCl

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.001

(b) Ar:HCl = 870:1

H2O 27 ML Ar 45 ML HCl@Ar Ar 45 ML Pt (111)

0.001

2950

2900

2850

2800

2750

2700

-1

Wavenumber (cm ) Figure 1. RAIR spectra of HCl in an Ar matrix at ~7 K in the absence of an external electric field. (a) Ar:HCl molar ratio of the HCl@Ar sample was 340:1. (b) Ar:HCl molar ratio was 870:1. The HCl@Ar films in the two samples were of different thickness, 90±3 ML (a) and 450±16 ML (b), in order to compensate for the different HCl concentrations of the samples. The inset shows a schematic drawing of the ice film capacitor structure containing a HCl@Ar film. The HCl monomer, dimer, and trimer peak positions are marked by dashed lines. Typical IR spectra have significant slopes due to multiple reflections in a thin film sample. Such base-lines have been subtracted in the spectra displayed in the figure. 4


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According to previous studies,22, 23, 35 the monomer peak at 2888 cm–1 corresponds to the R(0) (J’’ = 0 → J’ = 1) transition of HCl rotors inside the matrix and not the fundamental vibrational frequency of HCl (ν10 = 2871 cm–1). Unlike the previous observations made at higher temperatures, the P branch structure did not appear in the present spectrum because of the low temperature. With the effective rotational constant of 8.5–9.0 cm

–1

for HCl in the Ar matrix at 7 K,

23, 24

it is expected that more than 97

percent of the molecules are in the rotational ground state according to the Boltzmann distribution. The width of the R(0) peak is much broader for HCl in the matrix than that in the gas phase due to inhomogeneous spectral broadening.

35

An electric field was applied across the HCl@Ar sample by charging the H2O surface of the ice +

film capacitor with Cs ions.

20, 21

This led to the Pt metal surface becoming negatively charged by

induced electrons, as depicted in Figure 2. The spectral changes resulting from the applied electric field are displayed in Figure 2. First, the R(0) peak at 2888 cm–1 was blue-shifted in the presence of the applied field. Also, its intensity decreased with increasing field strength and disappeared almost completely at a field strength above ~6×107 V∙m–1 (between the green and blue spectra). Second, a new peak appeared at 2871 cm–1, which corresponded to the fundamental vibrational band center of HCl. The band center position is estimated from the R and P branch structures of HCl in an Ar matrix that appear at higher temperatures.23,

35

The shoulder structure of this peak at ~2868 cm–1

37

corresponds to the band center of the H Cl isotopomer. The intensity of the band center peak grew 7

–1

stronger with increasing field strength, and its position exhibited a vivid red-shift above ~8×10 V∙m . Third, another new (N) peak appeared at 2855 cm–1 with a lower intensity than that of the band center.

5



2.4 × 108 1.6 × 108 8.2 × 107 4.4 × 107 2.8 × 107 1.9 × 107 F = 0 (V⋅m-1)

N Alignment

Absorbance

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Pendular state

Perturbed rotation 0.001

2900

R(0) Q 2880

P(1) 2860 Wavenumber (cm-1)

Free rotation

+ + + + H2O 27 ML Ar 45 ML Ar 45 ML HCl@Ar Ar 45 ML -

2840

-

-

-

Pt (111)

Figure 2. Spectral evolution of the HCl monomer in an Ar matrix with increasing field strength. The expected positions of R(0), Q, and P(1) peaks are marked on the horizontal axis.

22, 23, 35

The position

-1

of new (N) peak, which appears initially at 2855 cm and red-shifts at higher field, is marked on the spectrum. The spectra at weak fields (≤ 4.4×107 V∙m–1; green line and below) were obtained with a thick sample (HCl@Ar film thickness of 450±16 ML, Ar:HCl ratio of 400:1). The high field spectra (≥ 8.2×107 V∙m–1; blue line and above) were obtained with a thin sample (HCl@Ar film thickness of 90±3 ML, Ar:HCl ratio = 340:1). Because of a limitation in the maximum voltage (< 45 V) across a sample in the present experimental setup, a thin sample was employed to achieve a high field strength. The different orientation states of HCl, indicated on the right-hand side of the figure, are a classification based on the changes in the spectral pattern (see text) and are not an impeccable definition of the states. The ice film capacitor structure is shown at the bottom right.

The appearance of the band center and the blue shift of R(0) peak in Figure 2 indicates that the applied field gradually changes the free rotation of HCl to pendulum motion bound by the electrostatic interaction potential of the dipole with the field. The field-induced change of a free rotor to a pendular 2,

state has been demonstrated previously, for example, for ICl seeded in supersonic molecular beams 5, 6

and a linear (HCN)3 complex in supersonic molecular beams.8 These studies showed that the

spectral change of linear molecules from a free rotor to pendular state is characterized by the appearance of a vibrational band center, and blue and red shifts of the R and P branches, respectively, 6


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away from the band center. These changes occur when the polarization of the infrared light and the applied field are in the same direction,9,

11

as in the present experimental geometry. The similar

behavior is observed in Figure 2, i.e., the coexistence of the R(0) and the band center peaks, and the blue shift of the R(0) peak. These changes indicate that HCl rotation is perturbed by the applied field. The quantum mechanical description for the transition from a linear free rotor to pendular state has been previously given in the literature,

6, 8

which may also apply to the present spectral changes. When

a molecule is situated in the Stark potential generated by an electric field along the z-direction (in laboratory space), the rotational quantum number (J) of the molecule is no longer a good quantum number, but the z-projection quantum number (M) is. The selection rule for the absorption of polarized light in the z-direction is ∆ = 0. In this case, there are two allowed rovibrational transitions for HCl in the perturbed rotor state at the low temperature (~7 K), going from the ground state (| = 0, ′′ = 0) to the | = 1, = 0 and | = 0, = 0 states, which produce the R(0) and band center peaks, respectively. The designation JF (subscript F for field) is used here instead of J because J is no longer a quantum number. As the field becomes even stronger (above ~6×107 V∙m–1), the molecules become essentially harmonically bound in the Stark potential with its dipole oriented preferentially along the field direction, and the R(0) structure fades away. In this pendular state, the degree of dipole alignment of HCl molecules along the field increases with the increase in the applied field strength. Meanwhile, the blue shift of R(0) in the perturbed rotor regime is caused by the Stark effect on the rotational states, which stabilizes the rotational ground state more than the first excited state, as calculated in the previous reports.9, 36 The reorientation energy of HCl in an electric field (u = µF with µ = 1.08 Debye for HCl) –1

instance, roughly 130 Jmol

7

37

is, for

–1

at F = 6×10 Vm . Since this energy is substantially larger than the –1

thermal energy (about 60 Jmol ) at the matrix temperature, it is reasonable that a majority of the HCl molecules are trapped in the Stark potential with its dipole lying along the field direction. Indeed, the R(0) structure almost disappeared at this field strength. In this regime, the intensity of the band center peak increased monotonously with increasing field strength, which indicated an increase in the light absorbance because of further alignment of the molecular axis along the direction of light polarization, which is parallel to the field direction. The rate of increase of the peak intensity diminished above 8

~2×10 Vm

–1

as the molecules approached asymptotically toward a perfect alignment with the field

direction. The red-shift of the band center of the dipole-aligned HCl molecules, appearing at high fields in Figure 2, occurs reversibly with respect to an increase or decrease in the field. This behavior is plotted in the upper panel of Figure 3, which shows a linear relationship between the reversible peak shift and 7

8

–1

the field strength in the range of 6×10 – 4×10 V∙m . The peak shift can be explained by the vibrational Stark effect of field-oriented HCl molecules (eq 2). ∆ = −

" #$

"

(Δ' ∙ ( + ( ∙ ∆+ ∙ ( + ⋯ )

(2)

*

7


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The Stark frequency shift (∆) is proportional to the vibrational Stark coefficient (or Stark tuning rate; Δ') and the field component in the direction of the H–Cl bond to a first approximation. The secondorder term related to the differential polarizability (∆+) usually produces only a small effect on the frequency shift at the present field strength.

38, 39

The unidirectional frequency shift appears because of

the field-orientation of the molecules. In contrast, for randomly orientated molecules, the Stark shifts of the oscillators on ensemble average give rise to spectral broadening rather than a unidirectional 25, 38

frequency shift.

7

The plot shows that the data points fall nicely into a straight line above 6×10

–1

V∙m . As the magnitude of the frequency shift depends on the angle between the directions of the molecular dipole and the applied field, ∆ ∝ │Δ'│ ∙ │(│ ∙ cos 3, the observed linearity with reliability >85% indicates that the average tilt angle of the molecules from the field axis is

Electric Field Effect on Condensed-Phase Molecular Systems. VI. Field

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