Volume 96, Issue 4, 15 February 1992
Index of content:
96(1992); http://dx.doi.org/10.1063/1.462046View Description Hide Description
This paper extends the McClain formulation of two‐photon absorption to show what observables can be independently determined in an emission experiment for which incident and scatteredphotonpolarizations are resolved. Interelectronic state coupling contributions and quantum interference can be extracted from the polarized emission spectrum by determining the three linearly independent contributions to the scattering intensity. This enhances the value of polarized emission spectroscopy as a tool for understanding transitions between coupled electronic states that occur during dissociation dynamics. We demonstrate that the three linearly independent contributions to the polarized emission spectrum can be determined by performing three polarization experiments, at least one of which must involve circularly polarized light. Furthermore, the three polarization experiments must be performed at a scattering angle other than π/2. We also present sample calculations on a model of the methyl iodide system which serve as an illustration of the general theory and demonstrate the sensitivity of this technique to the details of the potential‐energy surfaces.
96(1992); http://dx.doi.org/10.1063/1.462047View Description Hide Description
The ∼0.1 Å error in the inner wall of the experimental Rydberg–Klein–Rees (RKR) potential for the Na2 a 3Σ+ u state [L. Li, S. F. Rice, and R. W. Field, J. Chem. Phys. 8 2, 1178 (1985)], detected by Jenč and Brandt using the reduced potential curve (RPC) method [F. Jenč and B. A. Brandt, J. Chem. Phys. 9 1, 3287 (1989)], was due to neglect of centrifugal distortion effects and n o t due to a fundamental flaw in the LeRoy–Bernstein near‐dissociation expansion (NDE)G(v) and B(v) fitting expressions used to generate it. The NDE expressions, in fact, are p r e f e r a b l e to the more traditional Dunham expansions of B(v) and G(v) for the fitting of sparse data sets. We have refined the previously published RKR curve to include centrifugal distortion effects w i t h o u t i n c l u d i n g a d d i t i o n a l d a t a. The results are in excellent agreement with theoretical predictions, and are probably accurate to ±0.01 Å up to energies within 5 cm− 1 of dissociation. The principal constants are T e +Y 0 0=5848.48(22) cm− 1, ω e =24.15(2) cm− 1, B e =0.0562(1) cm− 1, r e =5.011(9) Å, and D e =175.76(35) cm− 1.
96(1992); http://dx.doi.org/10.1063/1.462048View Description Hide Description
Microwave measurements of rotational transitions in cyclopentadienyl manganese tricarbonyl were made using a Flygare–Balle type pulsed beam Fourier transformmicrowave spectrometer operating in the 4–14 GHz range. Ninety‐six hyperfine transitions were assigned for this prolate symmetric top for the rotational transitions J=2→3, 3→4, 4→5, 5→6, and 6→7. Molecular constants obtained from the analysis of the spectrum are B=828.0333(6) MHz, D J =0.088(9) kHz, D JK =−0.04(3) kHz, eQq aa (Mn)=68.00(4) MHz, C bb (Mn)=−5.5(8) kHz. The distortion parameter D J for CpMn(CO)3 is compared to other D J values for similar type transition metal complexes.
Ultraviolet photoelectron spectroscopy and photofragmentation studies of excess electrons in potassium iodide cluster anions96(1992); http://dx.doi.org/10.1063/1.462049View Description Hide Description
Ultraviolet photoelectron spectroscopy (UPS) and photofragmentation data for (KI)− n , K(KI)− n , and K2(KI)− n (n up to 13) are presented. The excess electrons in these clusters are all loosely bound with vertical detachment energies below 1.6 eV, and the photofragmentation data correlated well with the UPS results. Our results for the stoichiometric clusters are in good agreement with theoretical predictions, and the ground state geometry of these clusters are discussed. For the excess potassium cluster series, we suggest the possible existence of electron spin pairs in some of these cluster anions.
Microwave spectrum of alkali metal tetrahydroborate. II. Rotational transitions in the ground and excited vibrational states and molecular structure of LiBH496(1992); http://dx.doi.org/10.1063/1.462050View Description Hide Description
The rotational spectrum of LiBH4 was observed in the millimeter‐wave region using a high‐ temperature absorption cell. The observed spectrum of LiBH4 showed the pattern of a symmetric top molecule: strong and weak for K=3n and 3n±1, respectively, because of the nuclear spin statistical weight for C 3v symmetry. The rotational constants and centrifugal distortion constants for 7Li11BH4, 7Li10BH4, 6Li11BH4, and 6Li10BH4 species were determined. Four observed rotational constants gave r s (Li–B) to be 1.939 38±0.000 10 Å, which, combined with the assumption that θ(H b –B–H t )=113 °, led to r(B–H b ) and r(B–H t ) to be 1.256±0.015 Å and 1.216∓0.015 Å, respectively, where the signs should be taken in the stated order. The uncertainties of r(B–H b ) and r(B–H t ) are mainly due to that of θ (H b –B–H t ), which is estimated to be ∓1.5 °. This bond length obtained for Li–B is much shorter than the reported value in crystal; 2.47 Å. The bond lengths derived indicate that LiBH4 has a tridentate molecular structure with three bridging hydrogen atoms as in the case of NaBH4, in agreement with predictions by ab initio calculations. Two sets of vibrational satellites were observed and analyzed for both 7Li11BH4 and 7Li10BH4. These satellites were assigned to the nondegenerate Li–B stretching and the doubly‐degenerate Li–B–H bending mode.
The CO–CO interactions in transition‐metal hexacarbonyls studied by Penning ionization electron spectroscopy: Cr(CO)6, Mo(CO)6, and W(CO)696(1992); http://dx.doi.org/10.1063/1.462051View Description Hide Description
Penning ionization electron spectra resulting from thermal collisions of He*(2 3 S) metastable atoms with gaseous Cr(CO)6, Mo(CO)6, and W(CO)6 were measured to probe the CO–CO interactions. The energy splitting of the CO 5σ‐derived levels [I.P.(8a 1g )−I.P.(8t 1u )] was determined for the first time. Further, the spectra show that the through‐space interaction among the CO 5σ orbitals not only stabilizes the orbital energy of the CO 5σ‐derived 8a 1g molecular orbital, but also gives rise to a diffuse electron distribution. The CO–CO interactions in the transition‐metal (TM) carbonyls were compared to those in the CO overlayer formed on TM surfaces.
96(1992); http://dx.doi.org/10.1063/1.462052View Description Hide Description
Picosecond time dependent fluorescence method has been used to measure the rotational reorientation times (τ r ) of three kinds of dye probes—oxazine 720 (a monocation), nile red (neutral but polar), and resorufin (a monoanion)—in a series of binary mixtures of water–amides, water–dipolar aprotics, and water–alcohols at 298 K. Most of the binary mixtures are characterized by the fact that at a particular composition (between 25% to 40% of the organic solvent in water), the viscosity (η) of the solution reaches a maximum value that is higher than the viscosities of the two solvents. As a consequence, the viscosity profile of the solution as a function of the organic solvent exhibits a bivaluedness, the extent of which is more, if the liquids are nearly isoviscous. The dielectric properties of the solution also change across the composition range. All the dyes show a near linear behavior of τ r vs η in formamide–water, N‐methylformamide–water, and methanol–water mixtures. A dual‐valued profile for τ r vs η is obtained for the cation oxazine 720 in the three dipolar aprotic mixtures (N, N‐dimethylformamide–water, N, N‐dimethylacetamide–water, and dimethylsulphoxide–water), with the rotational reorientation times being higher in the organic solvent‐rich zone, compared to the corresponding isoviscous point in the water‐rich zone. However, the anion resorufin shows a bivalued profile of τ r vs η only in dimethylsulphoxide–water mixtures, while the neutral nile red shows a linear behavior in all the dipolar–aprotic water mixtures. A hook‐type profile of τ r vs η is seen for the anion resorufin in ethanol–, 1‐propanol–, and 2‐propanol–water mixtures and for the cation oxazine 720 in 1‐propanol– and 2‐propanol–water mixtures; but nile red shows no departure from linear behavior even in alcohol–water mixtures.
The rotational dynamics of the cation oxazine 720 in dipolar aprotic–water mixtures is explained in terms of solvation since the dielectricfriction is minimal in these solutions. While the concept of dielectricfriction model seems to be adequate to understand the nonhydrodynamic behavior of the oxazine 720, the additional contribution due to solvation effects has to be necessarily taken into account to explain the rotational dynamics of the anion resorufin in alcohol–water mixtures.
96(1992); http://dx.doi.org/10.1063/1.462053View Description Hide Description
The 0–0 excitation energy of a 1 A g state of 2,2’‐bithiophene has been determined by measuring the two‐photon fluorescence excitation spectrum of a dilute solution of this molecule in crystalline n‐hexane at 77 K. Because the 0–0 energy is what would have been predicted by extrapolating previously measured α,ω‐dithienylpolyene 2 1 A g 0–0 energies to zero polyene chain length, it is assigned to the 2 1 A g state. The 0–0 band is centered at 36 173 cm− 1, approximately 6570 cm−1 above the 0–0 of the 1 1 A g to 1 1 B u transition. This order of bithiophene excited singlet states is opposite to that of the linear polyene with the same number of double bond but may reverse for chains longer than six repeat units.
96(1992); http://dx.doi.org/10.1063/1.462054View Description Hide Description
Microwave and far‐infrared spectra of the H3N–HOH dimer have been recorded from 36 to 86 GHz and 520 to 800 GHz with a planar supersonic jet/tunable laser sideband spectrometer. The a‐type pure rotational microwave data extend the previous m=0, K=0 A symmetry manifoldmeasurements of Herbine and Dyke [J. Chem. Phys. 8 3, 3768 (1980)] to higher frequency and also provide an additional set of microwave transitions in the m K=+1 E symmetry manifold. Two sets of five b‐type rotation–tunneling bands, one set shifted from the other by an approximately constant 113 MHz, have been observed in the far infrared. The splitting into two sets arises from water tunneling, while the overall band structure is due to internal rotation of the ammonia top. Nonlinear least‐squares fits to an internal rotor Hamiltonian provided rotational constants, and an estimation of V 3=10.5±5.0 cm−1 for the barrier height to internal rotation for the NH3 monomer. A nonlinear e q u i l i b r i u m hydrogen bond is most consistent with the vibrationally averaged rotational constants; with the angle cos−1[〈λ z 〉] determined from 〈λ z 〉, the projection of the ammonia’s angular momentum onto the framework; and with the nitrogen quadrupole coupling constants of Herbine and Dyke. The water tunneling splitting and observed selection rules place constraints on the barrier height for proton exchange of the water as well as the most feasible water tunneling path along the intermolecular potential energy surface. An estimated barrier of ∼700 cm−1 is derived for the water tunneling motion about its c axis.
96(1992); http://dx.doi.org/10.1063/1.462002View Description Hide Description
The 846 nm band system of jet‐cooled 51V2 has been recorded using resonant two‐photon ionization spectroscopy, and is assigned as the A’ 3Σ− u ←X 3Σ− g band system. Both the Ω’=1 u ←Ω‘=1 g and Ω’=0+ u ←Ω‘=0+ g subbands of the 0–0 band have been rotationally resolved and analyzed. Although the transition could, in principle, correspond to a π* g ←π u or σ* u ←σ g excitation, it is assigned as a δ* u ←δ g excitation. Either of the other two possibilities makes it difficult to rationalize the small r e d u c t i o n in bond length which accompanies the excitation [r e (X 3Σ− g )=1.77 Å, r 0(A’ 3Σ− u )=1.756 Å]. In addition, the second order spin–orbit splitting of the ground X 3Σ− g state is reanalyzed to predict the existence of a 1Σ+ g state, arising from the same sσ2 g dσ2 g dπ4 u dδ2 g electronic configuration as the X 3Σ− g ground state, at an energy about 1821 cm−1 above the ground state.
This allows a state previously observed in an electronic resonance Raman effect 1860 cm−1 above the ground state to be positively identified as the sσ2 g dσ2 g dπ4 u dδ2 g , 1Σ+ g state, which is responsible for the anomalous spin–orbit splitting in the ground X 3Σ− g state. A similar analysis of the spin–orbit splitting of the A’ 3Σ− u state predicts the location of the sσ2 g dσ2 g dπ4 u dδ1 g dδ*1 u , 1Σ+ u state at 13 426 cm−1 above the V2 X 3Σ− g (0+ g ) ground state.
96(1992); http://dx.doi.org/10.1063/1.462003View Description Hide Description
The vibrational spectrum of the C5carbon cluster produced by trapping the products of the vacuum‐ultraviolet photolysis of 2‐methyl‐1,3‐butadiene, or the high‐temperature evaporation of graphite in Ar at ∼10 K, has been analyzed by Fourier‐transform spectroscopy and has resulted in the identification for the first time of the second infrared‐active stretching mode, ν4(σ u ), at 1446.6 cm− 1. The assignment is supported by extensive 1 3C isotopic data, and is in good agreement with the results of a b i n i t i o calculations.
Vibrational predissociation in argon complexes of 3‐amino‐s‐tetrazine and 3‐amino‐6‐methyl‐s‐tetrazine: Evidence for extreme mode‐selectivity96(1992); http://dx.doi.org/10.1063/1.462004View Description Hide Description
We have investigated the vibrational predissociation of argon complexes of 3‐amino‐s‐tetrazine (AT) and 3‐amino‐6‐methyl‐s‐tetrazine (AMT). Twelve vibrational levels of complexes of the form AT–Ar n , where n=1, 2, and 3, were examined. The dissociation rate of the binary AT–Ar complex varied from 3×106 s−1 to >4×108 s−1 depending on the initially excited mode, with the fastest rate corresponding to the lowest energy vibration. Even levels with vibrational energies over five times that needed to break the van der Waals bond still showed significant amounts of fluorescence from the unfragmented complex. Photodissociation of the AT–Ar2 complex can yield two different dissociation products, AT and AT–Ar. The ratio of these photoproducts varied greatly among different vibrational modes. The 16b 2 level of AT–Ar2 at +462 cm−1 gives primarily the AT photoproduct, while the 16a 2 level at +505 cm−1 gives exclusively the AT–Ar photoproduct. The two levels of AT–Ar3 which were studied both appear to show only partial fragmentation. We examined the basic spectroscopy of AMT using vibrationally resolved fluorescence excitation spectroscopy, and assigned a number of vibrational levels in both the ground and excited states. We then used this information to measure the branching ratios and dissociation rates for seven vibrational levels of AMT–Ar. These vibrational predissociation rates show strong qualitative differences from similar data for tetrazine–Ar and aminotetrazine–Ar complexes. Finally, these results were compared to similar work on other tetrazine derivatives to attempt to account for this strong mode‐selective behavior.
96(1992); http://dx.doi.org/10.1063/1.462005View Description Hide Description
A new near infrared emission spectrum of 4HeD has been recorded at 4.2 K during proton irradiation of a mixed sample of helium gas and solid deuterium. The upper state of the transitions is an admixture of the accidentally degenerate D 2Σ+(v=0) and C 2Σ+(v=3) levels. The lower state is the previously unobserved A 2Σ+(v=2) level. The observation of this level allows for evaluation of the rotational constantsB ‘ 2 and D ‘ 2 for the A 2Σ+ state, and for a better estimate of the vibrational parameters ω‘ e and ω‘ e χ‘ e .
Permanent dipole moments and two‐color multiphoton resonances in the two‐level molecule: The rotating wave approximation versus exact results96(1992); http://dx.doi.org/10.1063/1.462006View Description Hide Description
The interaction of two continuous wave electric fields (lasers) with a two‐level molecule, that has a nonzero difference d, between the permanent dipoles of the two states involved in a multiphoton two‐color transition, is studied in the rotating wave approximation and by using exact methods. Analytical rotating wave approximation results for the time‐dependent populations of the molecular states and for the resonance profiles are derived and used to help discuss the effects of permanent dipole moments in the two‐color absorption problem. Several examples of multiphoton, two‐color, two‐level resonance profiles calculated in the rotating wave approximation and by using exact techniques, are employed for illustrative purposes. They are used to help quantify the conditions for the validity of the two‐color rotating wave approximation, which are considerably more restrictive than the analogous conditions for the one‐color rotating wave approximation with d≠0, and to illustrate the interpretive and predictive nature of the rotating wave approximation results. For example, there are many multiphoton, two‐color transitions that are available if a molecule possesses permanent dipoles (d≠0). The analytic rotating wave approximation expression for the molecule‐two laser coupling can often be used to reliably estimate the field parameters of the two lasers that are required to optimize a given transition relative to others.
96(1992); http://dx.doi.org/10.1063/1.462007View Description Hide Description
The permanent electric dipole moments of CaOH and SrOH in their X 2Σ+, A 2Π3/2, A 2Π1/2, and B 2Σ+ states have been measured using the technique of supersonic molecular beam optical Stark spectroscopy. For CaOH the values obtained were μ(X 2Σ+)=1.465(61)D, μ(A 2Π1/2)=0.836(32)D, μ(A 2Π3/2)=0.766(24)D, and μ(B 2Σ+)=0.744(84)D, while for SrOH the values were μ(X 2Σ+)=1.900(14)D, μ(A 2Π1/2)=0.590(45)D, μ(A 2Π3/2)=0.424(5)D, and μ(B 2Σ+)=0.396(61)D. The results are compared with values from a recent a b i n i t i o calculation for CaOH and with the predictions of a semiempiricalelectrostaticpolarization model.
96(1992); http://dx.doi.org/10.1063/1.462845View Description Hide Description
The yttrium monochloride molecule (YCl) has been produced in a free jet molecular beam apparatus by chemical reaction in a laser‐produced plasma. The origin band of the C 1Σ+–X 1Σ+ system of YCl was probed at the sub‐Doppler resolution of 120 MHz using a ring dye laser to excite fluorescence.Spectra due to the two isotopomers 8 9Y3 5Cl and 8 9Y3 7Cl were obtained, and molecular constants determined. The following bond lengths were derived (8 9Y3 5Cl):r 0(X)=2.384 78(36) Å, r 0(C)=2.460 50(40) Å (2σ error bounds). Results for the ground state are in good agreement with those recently reported by Xin e t a l. [J. Mol. Spectrosc. 1 4 8, 59 (1991)]. The permanent electric dipole moments for both the X and C states were determined by performing molecular beam Stark spectroscopy on the lines P(1) and R(0), respectively. Values of 2.587(29) D (X state) and 3.258(36) D (C state) were obtained. Results are compared with a b i n i t i o predictions of the molecular parameters, and a molecular orbital interpretation of the bonding is presented.
96(1992); http://dx.doi.org/10.1063/1.462008View Description Hide Description
Perturbation theory is used to understand the experimentally observed stimulated emissionspectra of OH–Ar(X 2Π). A useful zero‐order Hamiltonian for an open‐shell van der Waals complex is presented, and the most important perturbation terms are identified: rotational decoupling ( j⋅s), Renner–Teller coupling (V̂2), and a Coriolis interaction (J⋅j). This treatment reveals those parts of the Hamiltonian which are responsible for various unusual features in the spectra of open‐shell complexes, such as the large parity splittings in certain vibrational bands and spin–orbit–induced predissociation of the OH–Ar(X 2Π) complex. The magnitude of the parity splitting is shown to be directly proportional to the change in the intermolecular potential when the odd electron in the free radical lies in or out of the O–H–Ar plane, the A’ and A‘surfaces. The measured splitting is used to infer the magnitude of the difference between the A’ and A‘ potential‐energy surfaces (∼12 cm−1) in the region sampled by the first excited bend.
96(1992); http://dx.doi.org/10.1063/1.462009View Description Hide Description
Rotational coherencespectroscopy has been used to measure the rotational constants of four isotopomers of phenol dimer and a single isotopomer of p‐cresol dimer. From the results of these measurements, together with spectroscopic results reported by others, a geometry for phenol dimer is deduced. The species is found to be bound by an O–H⋅⋅⋅O hydrogen bond. The orientation of the phenyl moieties is such that they make maximal contact consistent with the constraints imposed by the hydrogen bond and by the van der Waals radii of the atoms. This geometric feature is cited as evidence for the significance of aromatic–aromatic attraction in the intermolecular interaction between the phenols.
Two‐color (2+1’) multiphoton ionization threshold photoelectron study of the Ar–NO van der Waals complex: Observation of intermolecular vibrational progressions of the Ar–NO+ cation96(1992); http://dx.doi.org/10.1063/1.462010View Description Hide Description
Two‐color (2+1’) multiphoton ionization high‐resolution threshold photoelectron spectroscopy has been applied to the Ar–NO van der Waals complex in a supersonic free jet. The resonant ionization process studied may be expressed by Ar–NO(X 2Π1/2,v‘=0) +2hν1→Ar–NO*(C 2Π,v’)+hν2→Ar –NO+(X 1Σ+,v +). Two strong anharmonic vibrational progressions have been observed in the threshold photoelectron spectra having frequencies of 79 and 94 cm−1. These vibrations have been assigned to the intermolecular bending and stretching motions of the (Ar–NO)+ complex cation. From the threshold photoelectron spectra, the adiabatic ionization potential of Ar–NO has been determined as 73 869±6 cm−1 (86 cm−1 higher than previously reported), and the dissociation energy of the (Ar–NO)+ cation has been calculated as 951 cm−1. The structural change of Ar–NO that occurs on photoionization has been calculated with the aid of simple Franck–Condon calculations, which suggest that the intermolecular bond distance of Ar–NO decreases by 1.03 Å, while the O–N–Ar angle decreases by 11.3°.
The sensitivity of absorption experiments with phase locked ultrashort pulses to the excited state potential energy surface96(1992); http://dx.doi.org/10.1063/1.462011View Description Hide Description
It is often believed, with some justification, that the excitation of a molecule with an ultrashort pulse generates wave packets whose behavior is well approximated by classical mechanics. Therefore, it seems that such pulses are not particularly useful for accurate studies of quantum dynamics of an excited molecule; at least not when compared to cw spectroscopy. In this paper we show that this shortcoming can be overcome if one studies the emission of a molecule which absorbs one photon by interacting with two phase locked ultrashort pulses. The excited state population is affected by quantum interference and the dependence of the emission on the delay time between the pulses is very sensitive to small changes in the excited state potential. This sensitivity is as high at room temperature as it is at zero degrees Kelvin. The use of phase locked pulses can distinguish between two potential energy surfaces for which the traditional pump–probe experiments give practically identical results. These statements are documented by exact numerical calculations for the case of the I2 molecule and by analysis. We also discuss the reasons why the rotational motion affects absorption from two phase locked pulses at very short delay times between the pulses.