New rovibrational bands of the Ar–H2O complex at the ν2 bend region of H2O

Accepted Manuscript New rovibrational bands of the Ar-H2O complex at the ν2 bend region of H2O Xunchen Liu, Yunjie Xu PII: DOI: Reference:

S0022-2852(14)00090-3 http://dx.doi.org/10.1016/j.jms.2014.04.005 YJMSP 10438

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Journal of Molecular Spectroscopy

Please cite this article as: X. Liu, Y. Xu, New rovibrational bands of the Ar-H2O complex at the ν2 bend region of H2O, Journal of Molecular Spectroscopy (2014), doi: http://dx.doi.org/10.1016/j.jms.2014.04.005

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New rovibrational bands of the Ar-H2 O complex at the ν2 bend region of H2 O Xunchen Liu1 , Yunjie Xu∗ Department of Chemistry, University of Alberta, Edmonton, Canada, T6G 2G2

Abstract Several new rovibrational bands of the Ar-H2 O complex at the ν2 bend region of H2 O have been recorded using a multipass infrared absorption spectrometer with a quantum cascade laser. For five of these bands, the upper states have been unambiguously assigned to the Σ and Π levels of the (212 ) and ns = 1, (101 ) states of the complex. These new internal rotation states have similar energy patterns as those detected previously in the ground vibrational state of water, indicating that the ν2 excitation of the H2 O subunit exerts only minor perturbation on these states. Another five new bands belonging to the Ar-H2 O complex have also been observed with their upper states tentatively labeled as the “ns = 1, (110 )” and “ns = 1, (212 )” states. Keywords: Rare-gas water complex, quantum cascade laser, ro-vibrational spectrum, jet-cooled molecular complex, large amplitude motion

1. Introduction Water is an universal solvent for ionic, polar and non-polar molecules. Rare gas (Rg) atoms have substantial solubility in water. At 20◦ C, the solubility of Xenon is 108 cm3 /kg and that of Krypton is 60 cm3 /kg. This decreases to 34 cm3 /kg for Argon, which is still substantially higher than for hydrogen, nitrogen, carbon monoxide, methane, and is about the same for oxygen[1]. Molecular ∗ [email protected] 1 Current address: Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Canada, T6G 2V4

Preprint submitted to Journal of Molecular Spectroscopy

April 29, 2014

level understanding of the hydration of Rg atoms requires detailed knowledge about the structure and dynamic of the Rg-H2 O binary complex supported on the multidimensional intermolecular potential energy surface (PES). This is a prototypical model of hydrophobic interactions and is prerequisite to a detailed understanding of the more complicated interactions between water and biological molecules. With the flat minima and low barriers of the PES, the floppy weakly-bonded Ar-H2 O complex has large amplitude motions (LAM) which access portions of the PES far from the equilibrium[2]. High resolution infrared spectra of such complexes directly probe their rovibrational states which depend in a very sensitive way on the intermolecular PES[3–8]. Experimentally, the Ar-H2 O complex was extensively studied using far-IR spectroscopy by Cohen et al.[9–11], Suzuki et al.[12], and Zwart et al.[13]. The vibrational ground state of the Ar-H2 O complex was studied in the microwave region by Fraser et al.[14] and Germann et al.[15]. High resolution infrared spectra of Ar-H2 O were measured at the 3 µm region by Lascola et al.[16] and Nesbitt et al.[17] and at the 6 µm region by Weida et al.[18]. Empirical PES (AW2) was extracted from these spectroscopic data[11, 19–21]. Ab initio calculations of the PES of Ar-H2 O were carried out by Bulski et al.[22], ChaÃlasi´ nski et al.[23], Tao et al.[24], and Hodges et al.[25]. Recently, Makarewicz[26] constructed a highly accurate analytical Ar-H2 O PES at the CCSD(T)/CBS level of theory. The well depth is very close to that obtained from the experimental fitted AW2 potential. Furthermore, the barriers to the in-plane rotation are consistent with the previous ab initio studies of the other Rg-H2 O systems such as Xe-H2 O and Kr-H2 O but contrary to the AW2 potential. Clearly, a larger set of spectroscopic data would be desirable to improve the empirical PES and test the ab initial PES. Separation of rotation and vibration of such complexes is poor and there are usually strong rovibrational couplings. The rovibrational energy levels of the floppy Ar-H2 O complex are best described by the nearly free internal rotor model suggested by Hutson[19]. The assumption is that the interacting monomers are not significantly affected by the weak van der Waals forces. 2

Therefore, the rovibrational term values of Ar-H2 O can be represented by a pseudo-diatomic molecule Hamiltonian[10]: Evib-rot = ν + B[J(J + 1) − K 2 ] − D[J(J + 1) − K 2 ]2 + H[J(J + 1) − K 2 ]3 The pseudodiatomic rotational term to describe the end-over-end rotation of the complex is essentially the same as that of a linear polyatomic molecule. The vibrational energy terms include the “intramolecular” vibration of the H2 O subunit and the “intermolecular” van der Waals stretching between the H2 O subunit and the Ar atom (ns ). The nearly free internal rotation or vibration rotation tunneling states of the H2 O subunit in the complex are correlated to the free H2 O molecule rovibrational energy state JKa Kc . The internally rotating H2 O subunit contributes an effective moment of inertia and an angular momentum component K along the intermolecular axis of the complex[16]. Therefore, the internal rotation states of the complex is further characterized by the magnitude of the projection of this angular momentum, labeled as Σ, Π, ∆, · · · states for K = 0, 1, 2, · · · respectively, with Coriolis coupling term mixing the Σ and Π levels. The previously measured internal rotation states of the Ar-H2 O complex are shown in Fig. 1. Here we report our quantum cascade laser (QCL) measurements in the frequency region from 1625 cm−1 to 1670 cm−1 . Assignments of several new bands are described in this paper. Our measurements provide further high resolution mid-IR data of the Ar-H2 O complex with more information about the structure and dynamic of this complex. 2. Experiments To measure the high resolution mid-IR spectrum of Ar-H2 O complex, we use a pulsed slit-nozzle multipass direct absorption spectrometer with a quantum cascade laser (QCL). The experimental setup was reported in details before[27, 28] and is briefly described as follows. The Ar-H2 O complex is generated in a supersonic jet expansion with approximately 0.2% of H2 O and 3% of Ar in

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8 Bar of Neon (Praxair) backing gas through a homemade 0.025 mm×40 mm slitnozzle. An astigmatic multipass absorption cell (Aerodyne Research) aligned to a 366-pass configuration is mounted in the vacuum chamber evacuated by a 8000 L/s oil diffusion pump (Leybold, DIP8000) backed by a combination of a roots blower (Leybold, Ruvac WAU251) and a rotary pump (Leybold, Trivac D65B). The 3" astigmatic mirrors are heated to 60◦ C to eliminate pump oil deposition on the mirror surface. Since the multipass alignment is very sensitive to the mirror condition, two temperature controllers are employed to stabilize the temperature of each mirror and a micrometer is installed to allow the fine adjustment of the mirror distance under vacuum condition. An external-cavity room temperature mode-hop-free QCL (Daylight Solutions) is used to scan the frequency region from 1630 cm−1 to 1660 cm−1 . Fine tunning of the laser frequency covering ∼1 cm−1 with a repetition rate of 100 Hz is achieved by a piezoelectric transducer (PZT) attached to the external laser cavity. Since the laser wavelength is mechanically modulated by the PZT with a sine wave and there is a frequency drift between scans, frequency calibration is performed after each laser sweep before co-adding. During each scan, the signals from the supersonic jet, from a solid Germanium etalon, and from a Herriott multipass reference cell which contains a diluted mixture of NH3 and H2 O gas are recorded using three Mercury Cadmium Telluride (MCT) detectors simultaneously to allow frequency calibration using the HITRAN database[29]. The accuracy of the frequency calibration is around 0.001 cm−1 . Such a “on the fly” calibration procedure can efficiently remove the noticeable sweep-to-sweep frequency drift to enable averaging of the Doppler limited jet signals without noticeable broadening of the peaks. 3. Results and Discussion In total, more than ten new rovibration bands related to the new internal rotation states of Ar-H2 O are observed. The carrier of these bands can be confidently attributed to the Ar-H2 O complex based on the experimental condition used and the distinguishable diatomic like band structure with unique B values 4

of about 0.1 cm−1 . The measurements are also aided by checking the Ar-H2 O bands from the previous measurements by Weida and Nesbitt[18]. In the following section, we first discuss the new bands with the upper states unambiguously assigned to the Σ and Π levels of the (212 ) and ns = 1, (101 ) states with v2 = 1 state of water, which has been measured in the far-IR region. The identities of the other new bands are less certain since the upper states have not been measured in the far-IR region. We tentatively “label” the upper states of these transitions to the Σ and Π levels of the ns = 1, (212 ) and ns = 1, (110 ) states. The ν2 bending bands of Ar-D2 O complex have also been studied by Li et al.[30] and Stewart et al.[31] in the 1190-1200 cm−1 region where the Σ(111 ) ← Σ(000 ), Π(111 ) ← Σ(000 ), Π(110 ) ← Σ(101 ), and Π(101 ) ← Σ(101 ) bands have been assigned and analyzed. Stewart et al.[31] observed two new Π ← Σ bands and tentatively assigned them as Π(202 ) ← Σ(111 ) and Π(211 ) ← Σ(202 ) bands. In the measurement of Ar-H2 O complex, these two new bands are also observed. The lower levels of these bands are Σ(101 ) or Π(101 ) states of the ortho ArH2 O or Σ(000 ) level of the para Ar-H2 O. These are the internal rotation states that are substantially populated in a supersonic jet expansion as confirmed by the microwave studies[14, 15]. It is quite unlikely that high lying Σ(111 ) and Σ(202 ) states of the para species can be substantially populated, under the relatively “warm” jet condition suggested by Stewart et al.[31]. In that case, the Π(110 ) state of ortho species should also be populated and observed in previous studies. Therefore, we tentatively assign these bands as the candidates for ns = 1, Π(110 ) ← Σ(101 ) band. In addition to these two new bands, several other new bands were also observed in the our measurement. 3.1. ns = 1, Σ(101 ) and ns = 1, Π(101 ) states In the previous mid-IR study[18], the ns = 1, Σ(101 ) ← Σ(101 ) and ns = 1, Π(101 ) ← Σ(101 ) van der Waals stretching transitions predicted around 1629 cm−1 and 1639.5 cm−1 were not found. In our experiments with the QCL spectrometer, we observe a weak band next to the previously measured Σ(111 ) ← Σ(000 ) band (Fig. 2). This band is a typical Π ← Σ band with a strong Q-branch, R(0)

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transition and no P (1) transition. The position of this band is very close to the prediction from far-IR data. The above observation further indicates that the ns = 1, Π(101 ) internal rotation state is not significantly affected by the high frequency H2 O bending vibration. The same can be applied to the ns = 1, Σ(101 ) state. At the predicted 1629 cm−1 region, a dense spectrum shown in Fig. 3 is observed. Besides two Π ← Σ bands overlapping around 1630 cm−1 and a few unassigned transitions between 1628.5 cm−1 and 1629.5 cm−1 , we can identify a Σ ← Σ band at 1629.3 cm−1 . The assignment of the two Π ← Σ bands at 1630 cm−1 are less certain, however the Σ ← Σ band at 1629.3 cm−1 can be unambiguously assigned to the van der Waals stretching ns = 1, Σ(101 ) ← Σ(101 ) band. Transition frequencies and the residuals from least square fit of the ns = 1, Σ(101 ) ← Σ(101 ) and ns = 1, Π(101 ) ← Σ(101 ) bands are listed in Table 1. The corresponding bands have not been observed in the far-IR region, probably because these bands are from the less populated Π(101 ) level.

We can predict that the

ns = 1, Π(101 ) ← Π(101 ) and ns = 1, Σ(101 ) ← Π(101 ) bands are around 1628 cm−1 and 1618 cm−1 . The unassigned lines around 1628.5 cm−1 in Fig. 3 may be due to the high J transitions of the ns = 1, Π(101 ) ← Π(101 ) band. 3.2. Σ(212 ) and Π(212 ) states The 212 ← 101 transition of water monomer is at 55.702 cm−1 in the vibrational ground state and at 1653.267 cm−1 with the v2 = 1 excitation in the mid-IR region. In the previous far-IR measurement, Cohen et al. were able to record the Π(212 ) ← Σ(101 ) and the Σ(212 ) ← Σ(101 ) band in the ground vibratinal state of the complex[11]. The authors were only able to assign four lines in the Σ(212 ) ← Σ(101 ) band because of the expected strong mixing with ns = 1, Π(110 ), ns = 2, Π(101 ) or other internal rotation states which has so far not been observed. But none of these perturbing states was observed. Recently, a new band was observed by Verdes et al. at 1658.03 cm−1 and assigned to the Π(212 ) ← Σ(101 ) band[32]. In our attempt to identify the (212 ) internal rotation states, we first examine

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the band structure at 1646.5 cm−1 (Fig. 4). Clearly, this is a Π ← Π band with a weak Q-branch and strong P and R-branches starting from R(1) and P (2) respectively. All the transitions are split into doublets due to perturbation of the degenerated Π states. The shifts of the perturbed levels are so large that one P branch component is largely shifted from the other and one R branch turns over at J=6. This band is unambiguously assigned to the Π(212 ) ← Π(101 ) band. Since the Σ(101 ) state is about 11.5 cm−1 lower than the Π(101 ) state in the vibrational ground state, we expect to observe the Π(212 ) ← Σ(101 ) band to be 11.5 cm−1 to the blue of the Π(212 ) ← Π(101 ) band. In our measurement at 1658 cm−1 , the Π(212 ) ← Σ(101 ) band with a strong Q branch is observed (Fig. 5). The P and R-branches are severally distorted due to the strong Coriolis coupling between the Π(212 ) level and the Σ(212 ) level. To determine the position of the Σ(212 ) level, we preliminarily fit the e and f symmetry of the Π(212 ) level as two distinct levels with different band origin. It turns out that the perturbed e symmetry Π(212 ) level is higher than the f symmetry one. Therefore, the perturbing Σ(212 ) state should be lower than the Π(212 ) state. This is in agreement with the far-IR observation that the Σ(212 ) level is about 6.5 cm−1 lower than the Π(212 ) level[11]. Around 6.5 cm−1 to the red of the Π(212 ) ← Σ(101 ) band, we observe a typical Σ ← Σ band at 1652 cm−1 with similar intensity (Fig. 6). This is unambiguously assigned to the Σ(212 ) ← Σ(101 ) band. Transition frequencies and the residuals from the least square fit of the Σ(212 ) ← Σ(101 ), Π(212 ) ← Σ(101 ) and Π(101 ) ← Π(101 ) bands are listed in Table 2. Fitting of the rovibrational transitions is performed with Pickett’s SPFIT program[33].

The AABS software package for Assignment and Analysis of

Broadband Spectra[34] is used to aid the assignment and fitting of the spectrum. A global fit is performed to include the current measurement with the previous measurements of the microwave[14, 15], far-IR[9–11], near-infrared[16, 17], and previous mid-infrared data[18] to fit the (212 ) and ns = 1, (101 ) bands to the pseudodiatomic Hamiltonian with Coriolis coupling terms mixing the Σ and Π levels. Since these Coriolis effects vanish identically for J = 0, the origins and 7

B rotational constants are constrained to be equal for the e and f symmetry states in the spectroscopic fitting procedure while the higher-order centrifugal distortion constants D and H are allowed to vary independently to account for any additional couplings that are not included in the effective Hamiltonian. The transitions are weighted according to the square reciprocal of the experimental uncertainty: 5 kHz for the microwave data, 0.7 MHz for the far-IR measurements, 0.0005 cm−1 for the near-IR data, 0.00025 cm−1 for the previous mid-IR measurements and 0.001 cm−1 for our current measurements. In total, 423 transitions are included in the fit. The root-mean-square (RMS) error of the microwave and far-IR transitions is 0.6 MHz and the RMS error is 0.003 cm−1 for the infrared data, which is consistent with the experimental uncertainty. We note that the previous measurement of the far-IR transitions showed that the Σ(212 ) and Π(212 ) levels are affected by strong Coriolis coupling and this lead to large standard deviation of the fit[11]. In our analysis of the ortho transitions, the Σ(212 ) and Π(212 ) levels are not included in the overall fit. The fitted spectroscopic constants are listed in Table 3. 3.3. Tentatively assigned bands Besides the unambiguously determined bands associated with the Σ(212 ), Π(212 ), ns = 1, Σ(101 ), and ns = 1, Π(101 ) states, there are at least 5 additional bands observed in our measurements that can be attribute to the ArH2 O complex. An example of these bands is shown in Fig. 3. Two Π ← Σ bands overlapping around 1630 cm−1 can be identified with distinct P , Q, Rbranches. Here, these two bands are very close to the Π(110 ) ← Σ(101 ) band. They can be successfully fitted as other unambiguously assigned Ar-H2 O bands. B rotational constants, very close to the other internal rotation states of ArH2 O complex, can be obtained for these two Π levels. In the Ar-D2 O spectrum at 1191.5 cm−1 measured by Stewart et al., two very similar Π ← Σ bands structure near the Π(110 ) ← Σ(101 ) band were also observed. The structure of the two Q branches in the Ar-H2 O complex and Ar-D2 O complex are almost identical. These two bands were tentatively assigned as Π(202 ) ← Σ(111 ) and

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Π(211 ) ← Σ(202 ) bands by Stewart et al.. However, at the ∼1 K temperature of the supersonic jet expansion, only levels within a few cm−1 of the lowest level of each nuclear spin symmetry are populated. Therefore in the vibrational ground state, only the Σ(000 ) level of the para manifold is populated, since the next higer levels, i.e., the ns = 1, Σ(000 ) and the Σ(111 ) states are 30-40 cm−1 higher in energy. Among the states of the ortho manifold, both the Σ(101 ) and the Π(101 ) levels are populated, which are ∼11 cm−1 apart. Other states that are more than 10 cm−1 higher in energy are very unlikely to be substantially populated to generate readily observed vibrational bands. Based on the comparable intensity of these two bands, we determined that the lower states of these two bands cannot be the other high lying states such as Σ(111 ) and Σ(202 ). Interestingly, these two Π levels are just 1 cm−1 to the blue of the ns = 1, Σ(101 ) level, yet they were not reported by the far-IR measurements. We tentatively labeled to these two levels as “ns = 1, Π1,2 (110 )”. The other tentatively assigned bands measured are presented in the supplementary materials. In total, five such bands are observed with the upper state tentatively labeled as the “ns = 1, (110 )” and “ns = 1, (212 )” states of the ortho species and the “ns = 1, Π(111 )” state of the para species. We assume that there are strong couplings among many states which are accidently close in energy. Spectra of these bands and the list of the transitions are given in Fig. S1−S4 and Table S1. A global fit including these transitions with totally 522 transitions is performed and the spectroscopic constants of these tentatively assigned bands are given in Table S2. 4. Discussion Our study of the ns = 1, (101 ) and (212 ) internal rotation states shows that the positions of all the bands associated with these states can be accurately predicted by mapping the corresponding far-IR states directly to the v2 = 1 band of H2 O. This means that the internal rotation states have basically the same energy pattern in the vibrational ground and excited states of the H2 O subunit.

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For these internal rotation states, the coupling of the high frequency H2 O subunit bending with the low frequency internal rotation and intramolecular van der Waals stretching is small. In addition, we didn’t observe any predissociation broadening from the measurements, suggesting very little coupling between the H2 O bend and intermolecular coordinates which would lead to predissociation of the complex. As mentioned above, splitting of the Σ and Π level of the internal rotation states is a direct indicator of the anisotropy of the PES. In Table 4, we list the splittings of the internal rotation states observed in the far-IR region and in our current mid-IR measurements. Comparing the splittings of the (110 ) state on the ground and ν2 excitation states, there is 0.5 cm−1 increase of the splitting, which indicates stronger Coriolis and angular-radial coupling with v2 = 1 state of H2 O. This is in agreement with the study of the Ar-D2 O complex where the (111 ) internal rotation state shows a similar increase of the splitting in the v2 = 1 vibrational state[30]. On the other hand, the splitting of the ns = 1, 101 state decreases upon the ν2 excitation. It is also quite interesting to note that the splitting for the (212 ) state is significantly smaller than those of the (110 ) and (101 ) states. As discussed later, the abnormal decrease of the splitting might be in part due to the mixing of Π(212 ) level with a perturbing state, therefore pusing the Π(212 ) state to lower energy. Even taking into account of the perturbation, the splitting of the (212 ) internal rotation state is still estimated to be several wavenumber smaller than in the (110 ) and (101 ) states. As shown in Table 3, although the off-diagonal Coriolis terms for the ortho j = 1 levels are very close to the prediction of 2B value, the β value of the (212 ) and ns = 1, (101 ) states don’t agree with the prediction. This suggests that additional coupling states should be considered, which may be some of the tentatively assigned states shown in Fig. 1 or states yet to be measured. This shows that the approximations made in the nearly free internal rotor model fails as the internal rotation energies move to higher values. However, the (212 ) and ns = 1, (110 ) internal rotation states that are half of the depth of the PES (143 cm−1 ) can still be fitted by one off-diagonal Coriolis coupling term. The 10

inadequacy of this model is more evident when we consider those tentatively assigned new bands where it is challenging to correlate the new levels observed to the internal rotation states. Further theoretical calculation of the rovibrational bound states of the Ar-H2 O complex is crucial to resolve these ambiguities. It is interesting to analysis how the attachment of Ar atom affects the nearly free internal rotation of H2 O. As suggested by Weida and Nesbitt[18], the “center of gravity” of the Σ and Π levels for a particular internal rotation state represents the energy of the unperturbed internal rotation energy level from the first order perturbation theory. The average of the split Σ and Π states of the internal rotation states in both the ground and ν2 excited states of the H2 O subunit are listed in Table 5 and plotted in Fig. 7, together with the corresponding values of the free H2 O monomer. We arbitrarily fix the ground state (110 ) level of the Ar-H2 O complex to the corresponding value of free H2 O monomer to demonstrate the effect of the Ar atom attachment to the H2 O rotational energy levels. Clearly, the energy difference between the (212 ) and (110 ) states in the v2 =1 excitation state greatly decreases upon complexation with the Ar atom while that between the (110 ) and (101 ) states increases on the H2 O ground state. The abnormal decrease of the energy of the (212 ) state indicates that it is very likely that the Π(212 ) level is “pushed down” by mixing with another state. We estimate that the unperturbed Π(212 ) level and consequently the averaged (212 ) state should be several wavenumber higher in energy. Yet, this cannot completely explain the abnormally small gap between the (212 ) and (110 ) internal rotation state. So in the following discussion, we take the value of the “perturbed” Π(212 ) level as it is. Since the v2 =1 excitation has little effect on the internal rotation states, as shown by the almost identical energy gap for the (110 ) states in the ground and v2 = 1 excited states, we assume similar trend for other internal rotor states in the ground and v2 = 1 state of H2 O subunit. In terms of the effective “rotational constants”, the of the (212 ) and (110 ) states of the H2 O subunit in the Ar-H2 O complex are A+B+4C and A+B, respectively. Neglecting the centrifugal distortion constants, the C rotational constant estimated form the (212 )-(110 ) gap is 7.2 cm−1 , which is 2 cm−1 smaller 11

than the free H2 O value. The decrease of the C rotational constant indicates that the in-plane rotation of the H2 O subunit in the complex, which is subjected to the in plane rotation barriers, is hindered. The same argument can be applied to the increase of the (110 )-(101 ) gap, which is A − C. Using the C value obtained above, the A rotational constant is roughly estimated to be 30 cm−1 , which is ∼2 cm−1 larger than the free monomer value. It should be noted that the (110 )-(101 ) gap upon ν2 excitation for the Ar-H2 O complex is 0.58 cm−1 smaller than free H2 O, which was attributed to a red-shift of the H2 O subunit bending origin[18]. However, with a large change of the A and C constants as discussed in this study, determination of the band origin needs direct measurements of bands that have the other internal rotation states such as the Σ(000 ) level as upper levels. 5. Conclusion Five new rovibrational bands of the Ar-H2 O complex are measured and unambiguously assigned. Term values for the Σ(212 ), Π(212 ), ns = 1, Σ(101 ), ns = 1, Π(101 ) internal rotation states are accurately determined. These internal rotation states agree with the prediction from the far-IR study, indicating that the bending excitation of H2 O subunit in the complex has only minor effect on these internal rotation states. The decrease of the C rotational constant of the nearly free rotation of H2 O unit is attributed to the hindered in-plane rotation of H2 O upon complexation with Ar atom. The new measurements provide more information for the study of the high energy internal rotation states and potential energy surface of the Ar-H2 O complex. A number of new van der Waals vibrational states of the Ar-H2 O complex that cannot be properly associated to the internal rotation states of the complex. Tentative assignment of five additional new bands are also reported. Assignment of these bands requires considering more complicated rovibrational couplings.

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6. Acknowledgment This research was funded by the University of Alberta, Canada Foundation for Innovation, and the Natural Sciences and Engineering Research Council (NSERC) of Canada. We thank the referee for comments to improve the manuscript. XL. thanks Alberta Innovates and BMO financial group for studentships. YX. holds the Tier I Canada Research Chair in Chirality and Chirality Recognition. References References [1] A. Earnshaw, N. Greenwood, Chemistry of the Elements, Second Edition, 2nd Edition, Butterworth-Heinemann, 1997. [2] Z. Bacic, J. C. Light, Theoretical methods for rovibrational states of floppy molecules, Annual Review of Physical Chemistry 40 (1) (1989) 469–498. [3] D. J. Nesbitt, R. Naaman, On the apparent spectroscopic rigidity of floppy molecular systems, The Journal of Chemical Physics 91 (7) (1989) 3801. [4] R. E. Miller, The vibrational spectroscopy and dynamics of weakly bound neutral complexes, Science 240 (4851) (1988) 447–453. [5] R. E. Miller, Vibrationally induced dynamics in hydrogen-bonded complexes, Accounts of Chemical Research 23 (1) (1990) 10–16. [6] D. J. Nesbitt, High-resolution infrared spectroscopy of weakly bound molecular complexes, Chemical Reviews 88 (6) (1988) 843–870. [7] D. J. Nesbitt, High-Resolution, direct infrared laser absorption spectroscopy in slit supersonic jets: Intermolecular forces and unimolecular vibrational dynamics in clusters, Annual Review of Physical Chemistry 45 (1) (1994) 367–399.

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potential-energy surface for internal h[sub 2]O rotation, The Journal of Chemical Physics 95 (11) (1991) 7917–7932. [17] D. J. Nesbitt, R. Lascola, Vibration, rotation, and parity specific predissociation dynamics in asymmetric OH stretch excited ArH2O: a half collision study of resonant VV energy transfer in a weakly bound complex, The Journal of Chemical Physics 97 (11) (1992) 8096. [18] M. J. Weida, D. J. Nesbitt, High resolution mid-infrared spectroscopy of ArH2O: the v2 bend region of H2O, The Journal of Chemical Physics 106 (8) (1997) 3078. [19] J. M. Hutson, Atom–asymmetric top van der waals complexes: Angular momentum coupling in Ar–H[sub 2]O, The Journal of Chemical Physics 92 (1) (1990) 157–168. [20] R. C. Cohen, R. J. Saykally, Extending the collocation method to multidimensional molecular dynamics: direct determination of the intermolecular potential of argon-water from tunable far-infrared laser spectroscopy, The Journal of Physical Chemistry 94 (20) (1990) 7991–8000. [21] R. C. Cohen, R. J. Saykally, Determination of an improved intermolecular global potential energy surface for Ar–H[sub 2]O from vibration–rotation– tunneling spectroscopy, The Journal of Chemical Physics 98 (8) (1993) 6007–6030. [22] M. Bulski, P. E. S. Wormer, A. van der Avoird, Ab initio potential energy surfaces of ArH2O and ArD2O, The Journal of Chemical Physics 94 (12) (1991) 8096. [23] G. ChaÃlasi´ nski, M. M. Szcze´sniak, S. Scheiner, Ab initio study of the intermolecular potential of ArH2O, The Journal of Chemical Physics 94 (4) (1991) 2807.

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[24] F. Tao, W. Klemperer, Accurate ab initio potential energy surfaces of ArHF, ArH2O, and ArNH3, The Journal of Chemical Physics 101 (2) (1994) 1129. [25] M. P. Hodges, R. J. Wheatley, A. H. Harvey, Intermolecular potentials and second virial coefficients of the waterneon and waterargon complexes, The Journal of Chemical Physics 117 (15) (2002) 7169. [26] J. Makarewicz, Ab initio intermolecular potential energy surfaces of the water-rare gas atom complexes, The Journal of Chemical Physics 129 (18) (2008) 184310. [27] Y. Xu, X. Liu, Z. Su, R. M. Kulkarni, W. S. Tam, C. Kang, I. Leonov, L. D’Agostino, M. Razeghi, R. Sudharsanan, G. J. Brown, Application of quantum cascade lasers for infrared spectroscopy of jet-cooled molecules and complexes, in: Quantum Sensing and Nanophotonic Devices VI, Vol. 7222, SPIE, San Jose, CA, USA, 2009, pp. 722208–11. [28] X. Liu, Y. Xu, Z. Su, W. S. Tam, I. Leonov, Jet-cooled infrared spectra of molecules and complexes with a cw mode-hop-free external-cavity QCL and a distributed-feedback QCL, Applied Physics B 102 (3) (2010) 629–639. [29] The HITRAN 2008 molecular spectroscopic database, Journal of Quantitative Spectroscopy and Radiative Transfer 110 (9-10) 533–572. [30] S. Li, R. Zheng, Y. Zhu, C. Duan, Rovibrational spectra of the ArH2O and KrH2O van der waals complexes in the v2 bend region of D2O, Journal of Molecular Spectroscopy 272 (1) (2012) 27–31. [31] J. T. Stewart, B. J. McCall, Additional bands of the ArD2O intramolecular bending mode observed using a quantum cascade laser, Journal of Molecular Spectroscopy 282 34–38. [32] D. Verdes, H. Linnartz, Depletion modulation of Ar-H2O in a supersonic planar plasma, Chemical Physics Letters 355 (5-6) (2002) 538–542.

16

[33] H. M. Pickett, The fitting and prediction of vibration-rotation spectra with spin interactions, Journal of Molecular Spectroscopy 148 (2) (1991) 371– 377. [34] Z. Kisiel, L. Pszczolkowski, I. R. Medvedev, M. Winnewisser, F. C. De Lucia, E. Herbst, Rotational spectrum of transtrans diethyl ether in the ground and three excited vibrational states, Journal of Molecular Spectroscopy 233 (2) (2005) 231–243.

17

List of Figures 1

Internal rotation states of Ar-H2 O complex . . . . . . . . . . . . −1

2

Spectrum at 1640 cm

. . . . .

20

3

Spectrum at 1630 cm−1 , ns = 1, Σ(101 ) ← Σ(101 ) band . . . . . .

20

−1

, ns = 1, Π(101 ) ← Σ(101 ) band

19

4

Spectrum at 1645 cm

, Π(212 ) ← Π(101 ) band . . . . . . . . . .

21

5

Spectrum at 1658 cm−1 , Π(212 ) ← Σ(101 ) band . . . . . . . . . .

21

−1

6

Spectrum at 1650 cm

, Σ(212 ) ← Σ(101 ) band . . . . . . . . . .

22

7

The rovibrational energy levels of H2 O and Ar-H2 O . . . . . . .

22

List of Tables 1

Transitions of the ns = 1, (101 ) internal rotation states . . . . . .

23

2

Transitions of the (212 ) internal rotation states . . . . . . . . . .

24

3

Fitted spectroscopic constants . . . . . . . . . . . . . . . . . . . .

25

4

Energy difference between the Σ and Π levels . . . . . . . . . . .

26

5

Ar-H2 O internal rotation states . . . . . . . . . . . . . . . . . . .

26

18

Energy(cm-1)

3820 3800 3780 3760 3740 3720 3700 1720 1700 1680 1660 1640 1620 1600

para-Ar-H2O

ortho -Ar-H2O v3=1


(101)

(000) Current measurement

v2=1 (110) #
(110) #
(101)

100 Ground
(212) 80 (110) 60
(110) 40
(101) (101) 20 0


(212)

(212) ns= 1,
(101) ns= 1, (101)

ns= 1,
(101) ns= 1, (101)

“ns= 1,
(212)” “ns= 1, (212)” “ns= 1,
1,2(110)”

*

“ns= 1, (111)”

# (111) #
(111)

ns= 1, (000)

(111)


(111) (000)

ns= 1, (000)

Figure 1: Schematic drawing of the internal rotation states of Ar-H2 O complex. Previously observed Ar-H2 O bands from the microwave (thick black), far-IR (black), near-IR (blue), and mid-IR (green) measurements are indicated. New bands that are unambiguously assigned are shown in red, while those that are tentatively assigned are in magenta. The internal rotor states that were also identified in the Ar-D2 O studies are marked by # sign, whereas the states associated with the tentatively assigned bands observed in both the Ar-H2 O and Ar-D2 O studies are marked by ∗ sign.

19

 (111)   (000) 6 5 4 3 2 P(1) R(0) 1

2

3

4

5

7

ns= 1, (101)   (101) 8 5

1636.0

1636.5

1637.0

1637.5

4

3

1638.0 1638.5 (cm
1)

5

Q(1)

P(2)

1639.0

R(0) 1 2 3 4 5

1639.5

1640.0

1640.5

Figure 2: The ns = 1, Π(101 ) ← Σ(101 ) band of ortho Ar-H2 O and the Σ(111 ) ← Σ(000 ) band of para Ar-H2 O.

ns= 1,  (101)   (101) 6 5 4 3 P(2)

R(0) 1

“ns= 1, 2(110)   (101)” 7 6 5 4 3

“ns= 1, 1(110)   (101)” 7

1628.5

5

4

1629.0

P(2)

1629.5

P(2)

2

3

4

Q(1) 8 R(0) 1 Q(1)

5

2

3

6

4

5

10 R(0) 1

1630.0 20 (cm
1)

2

1630.5

3

4

5

6

1631.0

Figure 3: The complicated spectral feature of ns = 1, Σ(101 ) ← Σ(101 ) band, together with two overlapping Π ← Σ bands around 1630 cm−1 . The ns = 1, Σ(101 ) ← Σ(101 ) can be unambiguously assigned. The upper states of the two Π ← Σ bands are tentatively labeled as the “ns = 1, Π1,2 (110 )” states. The unassigned transitions red to 1629.0 cm−1 are probably high J transitions of the ns = 1, Π(101 ) ← Π(101 ) band.

Figure 4: The Π(212 ) ← Π(101 ) band. With a weak Q-branch, strong P and R-branches starting from R(1) and P (2) and doublet structure, this band can be identified as a Π ← Π band. Although this band was not measured in the far-IR region, ground state combination differences confirm the assignment.

Π(212 ) ← Σ(101 ) 14 13 11 9 7 5 3 Q(1) 8

1656.0

7

1656.5

6

5

1657.0

4

3

P(2)

R(0) 1

1657.5 21 1658.0 ν (cm−1 )

14 13 12 1110 2

1658.5

3

4 5 6 78 9

1659.0

1659.5

Figure 5: The Π(212 ) ← Σ(101 ) band. This is a typical Π ← Σ band with strong Q-branch and absence of P (1) transition.

 (212)   (101) 12

11

1648

10

9

1649

8

1650

7

6

(cm
1)

5

1651

9 8 765 4

3

2 P(1) R(0) 234

1652

1653

Figure 6: The Σ(212 ) ← Σ(101 ) band. This band was measured in the far-IR region, but only 4 lines were assigned. The condensed R-branch is due to a large difference between the perturbed upper and lower state rotational energy levels.

1700 1680

H2O v2=1

Ar-H2O

212

1660 1640

110

1620

101

Energy (cm 1)

1600 100 80

Ground 212

60 40 20

110 101

22

0 Figure 7: The rovibrational energy levels of H2 O and Ar-H2 O. The rovibrational energy levels are taken as the average of the split Σ and Π level of a particular internal rotation state of the nearly free rotating H2 O subunit in the Ar-H2 O complex. The ground (101 ) internal rotation state of the complex is arbitrarily fixed to the 101 state of H2 O.

Table 1: Observed ortho ArH2 O transitions of the ns = 1, (101 ) internal rotation states (in cm−1 ). Residuals (observed-predicted) from least squares fit in least significant digit. J 0 ← J 00 ns = 1, Σ(101 ) ← Σ(101 ) ns = 1, Π(101 ) ← Σ(101 ) 0←1 1629.1310(4) 1←2 1628.9548(0) 1638.9552(3) 2←3 1628.7968(3) 1638.7433(4) 3←4 1628.6568(4) 1638.5221(2) 4←5 1628.5348(3) 1638.2914(1) 5←6 1628.4309(16) 1 2 3 4 5 6 7 8

← ← ← ← ← ← ← ←

1 2 3 4 5 6 7 8

1 2 3 4 5 6 7 8

← ← ← ← ← ← ← ←

0 1 2 3 4 5 6 7

1639.3378(1) 1639.3088(2) 1639.2653(3) 1639.2072(1) 1639.1341(1) 1639.0455(1) 1638.9415(3) 1638.8205(2) 1629.5377(0) 1629.7686(10) 1630.0161(4) 1630.2831(9) 1630.5671(3) 1630.8683(1) 1631.1848(7) 1631.5089(2)

1639.5376(1) 1639.7141(1) 1639.8811(0) 1640.0382(3) 1640.1856(1) 1640.3225(1)

23

Table 2: Observed ortho Ar-H2 O transitions of the (212 ) internal rotation states (in cm−1 ). J 0 ← J 00 Σ(212 ) ← Σ(101 ) Π(212 ) ← Σ(101 ) Π(212 ) ← Πf (101 ) Π(212 ) ← Πe (101 ) 0←1 1652.2273(5) 1←2 1651.9959(11) 1657.5461(11) 1646.2053(3) 1646.1888(2) 2←3 1651.7329(7) 1657.3481(5) 1645.9761(150) 1645.9458(151) 3←4 1651.4334(5) 1657.1441(20) 1645.7634(56) 1645.7140(40) 4←5 1651.1047(4) 1656.9294(28) 1645.5408(16) 1645.4564(1) 5←6 1650.7456(3) 1656.6970(3) 1645.3009(10) 1645.1685(34) 6←7 1650.3593(2) 1656.4469(2) 1645.0560(16) 1644.8582(15) 7←8 1649.9489(4) 1656.1743(10) 1644.8077(14) 1644.5166(13) 8←9 1649.5172(5) 1655.8736(32) 1644.5508(29) 1644.1313(36) 9 ← 10 1649.0640(9) 1655.5347(4) 1644.2846(23) 1643.7229(77) 10 ← 11 1648.5868(20) 1644.0057(32) 11 ← 12 1648.0938(6) 1 2 3 4 5 6 7 8 9 10 11 12 13 14

← ← ← ← ← ← ← ← ← ← ← ← ← ←

1 2 3 4 5 6 7 8 9 10 11 12 13 14

1←0 2←1 3←2 4←3 5←4 6←5 7←6 8←7 9←8 10 ← 9 11 ← 10 12 ← 11 13 ← 12 14 ← 13 15 ← 14

1657.9326(10) 1657.9208(13) 1657.9031(14) 1657.8793(12) 1657.8495(7) 1657.8144(6) 1657.7736(6) 1657.7264(3) 1657.6719(3) 1657.6099(6) 1657.5391(3) 1657.4573(3) 1657.3623(14) 1657.2488(6)

1652.7040(7) 1652.7940(8) 1652.8511(1) 1652.8792(14) 1652.8792(4) 1652.8562(3) 1652.8094(3) 1652.7405(7) 1652.6488(1)

1658.1308(8) 1658.3201(13) 1658.5054(41) 1658.6745(8) 1658.8325(15) 1658.9685(3) 1659.0806(3) 1659.1654(25) 1659.2156(38) 1659.2284(39) 1659.2024(31) 1659.1362(8) 1659.0315(14) 1658.8926(10) 1658.7165(63) 24

1646.6026(2)

1646.5917(2)

1646.9741(12) 1647.1436(27) 1647.2917(172) 1647.4619(13) 1647.6084(9) 1647.7486(13) 1647.8792(23) 1647.9965(13) 1648.1069(22) 1648.2087(8) 1648.2982(6) 1648.3720(10) 1648.4255(18)

1646.9778(14) 1647.1477(50) 1647.3139(270) 1647.4061(8) 1647.4954(26) 1647.5529(27) 1647.5719(35) 1647.5501(38) 1647.4842(48) 1647.3784(10) 1647.2273(23) 1647.0233(30)

Table 3: Fitted spectroscopic constants from a global fit of the previously measured data with the unambiguously determined (212 ) and ns = 1, (101 ) bands from current measurement. Uncertainties (1σ) from least squares fit is in least significant digit. ν(cm−1 ) B(MHz) D(kHz) H(Hz) β(MHz) Ground Σe (101 ) 3014.788(28) 72.637(27) 1.341(98) 115.84(75) 4.9(35) 5901.65(78) e Πf (101 ) 11.428610(73) 2951.956(50) 135.67(25) 0a Σf (110 )

36.145679(73)

2953.247(44)

Πfe (110 )

21.162294(9)

3037.5951(90)

Πef (212 )

61.3(16)

Ground,ns =1 Σe (101 ) Πf (101 ) v3 =1 Σe (000 ) v2 =1 Σf (110 )

112.32(57) 51.327(97) 60.379(71)

-21.7(21) -12.55(24) -21.43(15)

3184(9711) 2821(2564)

0a 0a

0a 0a

33.998168(15) 44.728064(19)

2731.055(47) 2693.965(35)

115.5(11) 149.44(44)

-32.0(68) -10.1(14)

3737.8070(2)

2975.21(40)

68.3(64)

-136(27)

118.4(54) 64.9(12) 57.3(14)

-49(24) -26.7(35) -7.5(39)

5667(5)

-727(32) 1593(15) 142.3(29)

-1798(166) 2569(55) 0a

2799(30)

1633.9962(2)

2951.66(34)

Πef (110 )

1618.5117(1)

3033.16(11)

Σe (212 )

1652.4213(7)

2422.6(19)

Πef (212 )

1658.0307(6)

2831.03(55)

v2 =1,n=1 Σe (101 )

1629.3257(7)

3259.7(42)

Πef (101 ) a

-413(134) 345(71) 1639.4422(6) 2696.2(16) 140(24) Zero within uncertainty, and set to zero in final

25

-4640(1280) 0a 0a fit.

5762.31(42)

4985(66)

Table 4: Energy difference between the Σ and Π levels of the internal rotation states with the water subunit on its vibrational ground state and bending excitation v2 = 1. Note the splitting of the (212 ) state is substantially smaller compared to the other states. ns = 0 ns = 1 Ground (101 ) 11.428610 10.729896 (110 ) 14.983385 v2 =1 (101 ) 10.1165 (110 ) 15.4845 (212 ) 5.6094

Table 5: The Ar-H2 O internal rotation states determined from this study together with the corresponding free H2 O monomer values taken from Ref [29]. The internal rotation state energy is calculated as the average of the Σ and Π levels. H2 Oa Ar-H2 Ob ns = 0 ns = 1 Ground 101 0 0 33.648811 110 18.5773 22.939681 212 55.7020 v2 =1 101 1594.7629 1634.38395 110 1616.7115 1620.539695 212 1653.2670 1649.511695 a set to zero at 23.7944 cm−1 b set to zero at 5.714305 cm−1

26

•

Quantum cascade laser ro-vibrational spectra of Ar-H2O.

•

Five new bands with Σ(212), Π(212), ns=1 Σ(101), and ns=1 Π(101) states assigned.

•

Five additional new bands tentatively assigned.