M. J. Dick et al- High resolution laser excitation spectroscopy of the B^2-E-X^2-A1 transitions of calcium and strontium monoborohydride

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THE JOURNAL OF CHEMICAL PHYSICS 126, 164311 2007 ˜ ˜ High resolution laser excitation spectroscopy of the B 2E-X 2A1 transitions of calcium and strontium monoborohydride M. J. Dick Department of Physics, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada P. M. Sheridana and J.-G. Wangb Department of Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada P. F. Bernathb ,c Department of Physics, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada and Department of Che
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  High resolution laser excitation spectroscopy of the B ˜   2 E  - X ˜   2 A 1 transitionsof calcium and strontium monoborohydride M. J. Dick  Department of Physics, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada P. M. Sheridan a  and J.-G. Wang b   Department of Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada P. F. Bernath b  ,c   Department of Physics, University of Waterloo, Waterloo, Ontario N2L 3G1, Canadaand Department of Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada  Received 22 January 2007; accepted 14 March 2007; published online 27 April 2007  High resolution spectra of the B ˜  2  E  -  X  ˜  2  A 1 transitions of CaBH 4 and SrBH 4 have been recorded usinglaser excitation spectroscopy in a laser ablation/molecular jet source. Because of rotational coolingin the molecular jet and nuclear spin statistics, transitions arising from only the K   =1 ← K   =0, K   =2 ← K   =1, and K   =0 ← K   =1 subbands have been observed. For each molecule, an analysis of the data using 2  E  and 2  A 1 symmetric top Hamiltonians yielded rotational, spin-orbit, andspin-rotation parameters for the observed states. For both molecules the rotational constantscompare well with those calculated for a tridentate borohydride structure. A large reduction in thespin-orbit splitting and in the metal-ligand separation for each molecule indicates an increase in theamount of  d  atomic orbital character in the first excited 2  E  states of the monoborohydrides ascompared to the monomethyl derivatives. For each molecule no evidence of internal rotation of theBH 4− ligand was found.Achange in the magnitude and sign of the spin-rotation constant  1 confirmsan energy reordering of the first excited 2  E  and 2  A 1 states in both CaBH 4 and SrBH 4 as comparedto CaCH 3 and SrCH 3 . The data also suggest that the B ˜  2  E  1/2 rotational energy levels of CaBH 4 maybe perturbed by a vibronic component of the A ˜  2  A 1 state. © 2007 American Institute of Physics .  DOI:10.1063/1.2723097  I. INTRODUCTION CaBH 4 and SrBH 4 are the only members of the isoelec-tronic 2  p ligand family of molecules  CaF/SrF,CaOH/SrOH, CaNH 2  /SrNH 2 , and CaCH 3  /SrCH 3  1–6 thathave not yet been observed using high resolution spectros-copy. From the previous investigations of the molecules inthis series, the metal-ligand bonding has been found to belargely ionic and to occur between the metal cation and theheaviest atom of the negatively charged ligand. In contrast,the borohydride anion  BH 4−  , which possesses a tetrahedralstructure, cannot form a bond directly between the centralboron and metal atoms. 7,8 In this case, the metal-ligandbonding in the alkaline-earth borohydrides must occurthrough bridging hydrogens. As a result, three structures arepossible, monodentate  one bridging hydrogen, C  3 v symme-try  , bidentate  two bridging hydrogens, C  2 v symmetry  , andtridentate  three bridging hydrogens, C  3 v symmetry   struc-tures can be found in Ref.9  . In addition, the electronicstructure of the molecules in the 2  p ligand series has beenwell described in terms of a perturbation of the metal atomicorbitals by the ligand. The unique geometric configurationspossible for the alkaline-earth borohydrides may disrupt thisperturbation description and may have an affect on the ex-cited state electronic structure, which makes them of particu-lar interest for spectroscopic investigation.One group of metal borohydrides that have been the sub- ject of several studies, both theoretical and experimental, isthe alkali metal borohydrides. The experimental work 9–18 hasconsisted primarily of microwave spectroscopic investiga-tions in the gas phase. In these studies, the rotational andhyperfine parameters have been determined for the groundelectronic states of NaBH 4 , 9–12 NaBD 4 , 10,13 LiBH 4 , 10,14,15 LiBD 4 , 13 and KBH 4 . 11,13 Based on isotopic data, lithium andsodium borohydrides were found to possess the tridentatestructure in the ground state. Theoretical studies 19–28 havealso predicted this geometric configuration for the groundstate. Additionally, several of these theoretical studies havecalculated a low-energy barrier to the internal rotation of theligand, 19,22–25,27,28 in which the BH 4− cycles from one triden-tate minimum to another through a bidentate intermediatestructure. This phenomenon will manifest itself in theob-served rotational spectra as a doubling of spectral lines. 29 Inthe rotational spectroscopic studies of the alkali metal boro- a  Present address: Department of Chemistry and Biochemistry, Canisius Col-lege, Buffalo, New York, 14208-1098. b  Present address: Department of Chemistry, University of York, Heslington,York, YO10 5DD, UK. c  Author to whom correspondence should be addressed. Tel:  44-1904-434526; Fax:  44-1904-432516; Electronic mail: pfb500@york.ac.uk THE JOURNAL OF CHEMICAL PHYSICS 126 , 164311  2007  0021-9606/2007/126  16   /164311/10/$23.00 © 2007 American Institute of Physics 126 , 164311-1 Downloaded 14 Aug 2007 to 129.97.47.146. Redistribution subject to AIP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp  hydrides, this doubling of spectral lines was not resolved,indicating that the barrier to the internal motion of the BH 4− group is larger than that calculated.Studies involving the alkaline-earthborohydrides havebeen far less extensive. Pianalto et al. 30 have used laser ex-citation and dispersed fluorescence experiments in a Broidaoven to examine the A ˜  2  A 1 -  X  ˜  2  A 1 and B ˜  2  E  -  X  ˜  2  A 1 electronictransitions of CaBH 4 and SrBH 4 at low resolution. Fromtheir spectra, they suggested that these molecules possessed C  3 v symmetry and a tridentate structure similar to the struc-ture found later for the alkali borohydrides. Most interest-ingly, they found that the first excited 2  A 1 and 2  E  states ap-peared to be reversed in energy relativetothe corresponding states in the metalmonomethyls, SrCH 35,6 and CaCH 3 . 6,31,32 Subsequently, Ortiz 33 used electron propagator methods tocalculate the energy, vibrational frequencies, geometricstructure, and orbital character of several electronic states of CaBH 4 . From this work a tridentate structure was predictedfor CaBH 4 . However, the first excited 2  E  and 2  A 1 electronicstates were found to be ordered in energy the same as inCaCH 3 , contrary to the experimental work. In addition, Chanand Hamilton 34 have used density functional theory to calcu-late ground state geometries and vibrational frequencies forthe monoborohydrides of calcium and strontium. In theirwork they also found that the tridentate structure was thelowest in energy in the ground electronic state.In an attempt to further understand the geometric andexcited state electronic properties of CaBH 4 and SrBH 4 andto reconcile the discrepancy between theory and experimentfor CaBH 4 , we have recorded high resolution laser excitationspectra of the B ˜  2  E  -  X  ˜  2  A 1 transitions of both calcium andstrontium monoborohydride. For each molecule the rota-tional and fine structure parameters have been determined forthe ground and excited electronic states. These constantshave conclusively established that the first two excited elec-tronic states have reversed their order in energy relative totheir MCH 3 analogs. A discussion of the geometry, energyordering, and orbital character of the observed states will bepresented. In addition, a comparison of the observed spectro-scopic parameters with those of SrCH 3 and CaCH 3 will bedescribed. II. EXPERIMENT A laser ablation/molecular jet source was used to pro-ducetheborohydrides of calcium and strontium in the gasphase. 35 First, a gas mixture was introduced into a reactionnozzle via a pulsed valve. For this synthesis a mixture of 5%diborane in argon  Praxair  , at a backing pressure of 100 psi,was used as the reactant gas. Next, the third harmonic  355 nm  of a pulsed  10 Hz  neodymium-doped yttriumaluminum garnet laser  10 mJ/pulse  was used to vaporize ametal target rod  calcium or strontium  located in the nozzle.Inside the reaction region a high-energy plasma was formed.Here the diborane reacted with the metal vapor, producingthe metal borohydrides. The molecules then exited into thelow pressure chamber  1  10 −7 Torr  , forming a molecu-lar jet with a low rotational temperature  T  rot  4–6 K  . Aprobe laser was then sent perpendicularly through the expan-sion  15 cm downstream. The resulting molecular fluores-cence was collected by a photomultiplier tube, sent through apreamplifier  100  current  , and processed by a boxcar inte-grator. Bandpass filters  ±20 nm  were used to help attenuatestray radiation from the ablation laser and plasma emission.Initially, low resolution survey scans were obtained us-ing an argon-ion-pumped linear dye laser  linewidth  30 GHz  scanned at a speed of   50 cm −1  /min. DCM andpyridine 2 laser dyes were used to observe the spectral regionof interest  13 500–15 500 cm −1  with a typical maximumoutput power of   1 W  pump power  5 W  . A LABVIEW data acquisition program was used to plot the output signalsfrom the boxcar integrator versus the wave number of thelaser, which was obtained using a wavemeter  Burleigh WA-2500 Wavemeter Jr.  interfaced with the program.For the B ˜  2  E  -  X  ˜  2  A 1 transitions of both CaBH 4 andSrBH 4 , an argon-ion-pumped Coherent Autoscan 699-29ring dye laser system, operating with DCM laser dye, wasused to obtain high resolution spectra  linewidth  10 MHz,output power  1 W  pump power  8 W  . Generally, spec-tra were obtained in 5 cm −1 portions at a scan rate of 180 s/cm −1 and a data sampling interval of 10 MHz. Toachieve an adequate signal to noise ratio, several  2–4  por-tions were usually averaged together. The final spectra werethen calibrated using the line positions of I 2 obtainedfromsimultaneously recording its laser excitation spectra. 36 III. RESULTS Figure1shows low resolution survey spectra of the  A ˜  2  A 1 -  X  ˜  2  A 1 and B ˜  2  E  -  X  ˜  2  A 1 transitions of CaBH 4 andSrBH 4 . In each spectrum, three peaks are clearly present. For FIG. 1. Low resolution spectra recorded for the A ˜  2  A 1 -  X  ˜  2  A 1 and B ˜  2  E  -  X  ˜  2  A 1 transitions of SrBH 4  upper panel  and CaBH 4  lower panel  . For each mol-ecule the A ˜  2  A 1 -  X  ˜  2  A 1 transition was weaker in intensity and located at alower energy than the corresponding B ˜  2  E  -  X  ˜  2  A 1 transitions. This energyordering is in contrast to that of CaCH 3 and SrCH 3 , but is consistent withthe previous low resolution observation of calcium and strontiumborohydrides. 164311-2 Dick et al. J. Chem. Phys. 126 , 164311  2007  Downloaded 14 Aug 2007 to 129.97.47.146. Redistribution subject to AIP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp  CaBH 4 the separation between the first and second peaks is  600 cm −1 , while the second and third peaks are spaced by  70 cm −1 . The 70 cm −1 separation is in the range of thespin-orbit splitting previously measured for the A ˜  states of CaF, CaOH, and CaCH 3 . 1,2,6,31,32 This suggests that the sec-ond and third peaks are two spin-orbit components of anorbitally degenerate electronic state. This scenario is onlypossible if the molecule possesses a geometric configurationwith C  3 v symmetry. For a molecule having C  2 v symmetry,such as CaNH 2 , 37 no two of the first three excited statesshould be spaced by this  70 cm −1 separation exhibited bycalcium-containing molecules  C  2 v symmetry eliminates firstorder spin-orbit coupling  . Therefore, the wave number sepa-rations exhibited by the first three peaks for CaBH 4 suggestthat C  2 v symmetry is unlikely. A similar conclusion can bearrived at for SrBH 4 , where the second and third peaks arespaced by  200 cm −1 . This separation is consistent with thepreviously observed spin-orbit splitting in the A ˜  states of SrF,SrOH, and SrCH 3 . 1–3,5 As a result, for the borohydrides thefirst excited 2  A 1 state was found to lie lower in energy thanthe first excited 2  E  state, consistent with the previous obser-vations of the metal borohydrides. 30 It is also of interest tonote the decreased intensity of the A ˜  2  A 1 -  X  ˜  2  A 1 transitionsas compared to the B ˜  2  E  -  X  ˜  2  A 1 transitions in the spectra inFig.1.For each molecule the decreased signal strength was observed previously 30 and does not seem to be simply aneffect of the lower laser power. The decrease in intensity of the A ˜  2  A 1 -  X  ˜  2  A 1 transitions impeded the measurement of their high resolution spectra.Figure2shows the overall high resolution spectra ob-tained for both spin-orbit components of the B ˜  2  E  -  X  ˜  2  A 1 tran-sitions of CaBH 4 and SrBH 4 . The B ˜  2  E  -  X  ˜  2  A 1 transitions cor-respond to a p   ← s   promotion on the calcium or strontiumion. Transitions of this type should have the general appear-ance of a Hund’s case  a  2  –Hund’s case  b  2  + perpen-dicular transition of a corresponding linear molecule such asCaCCH 38,39 and SrCCH. 40 In this case two spin-orbit com-ponents, each with a well defined srcin and  1  B and  3  B spaced branches, are present. For SrBH 4 , the B ˜  2  E  3/2 -  X  ˜  2  A 1 component  lower left panel  indeed exhibits the appearanceof a perpendicular transition with  1  B and  3  B spacedbranches. The B ˜  2  E  1/2 -  X  ˜  2  A 1 component  upper left panel  ,on the other hand, exhibits a pattern that closely resembles aparallel transition with only  2  B spaced branches present.For the B ˜  2  E  -  X  ˜  2  A 1 transition of CaBH 4 , neither spin-orbitcomponent  top or bottom right panel  has the anticipatedperpendicular-like appearance nor a clear srcin.The energy level structure of both the B ˜  2  E  and X  ˜  2  A 1 states of the metal monoborohydrides closely resembles thatof the corresponding monomethyls. 5,31,32 These energy levelscan be described by considering two factors. First, the boro-hydrides are prolate symmetric top molecules belonging tothe C  3 v symmetry group. Therefore, their rotational energylevel structure is split into various sublevels, labeled by K   = K   R +   e  , where K   R and   e are the projections of the rota-tional angular momentum of the nuclei  R  and the elec-tronic orbital angular momentum  L  onto the symmetryaxis, respectively. Second, the spin angular momentum asso-ciated with the unpaired electron  S  must also be taken intoaccount. In the ground 2  A 1 state, the spin of the unpairedelectron interacts with the rotational angular momentum of the molecule to split each rotational level into two spin-rotation components, labeled by J  , the total angular momen-tum. These two components are identified by F  1  e parity  and F  2   f  parity  . In the 2  E  state, the spin of the unpairedelectron interacts with the electronic orbital angular momen-tum and gives rise to the 2  E  1/2  F  1  and 2  E  3/2  F  2  spin-orbitcomponents. Each rotational level in each spin-orbit compo-nent is further split into e and f  parity levels by the spin-rotation and Jahn-Teller interactions. This splitting is similarto  -type doubling in a linear molecule 41 and, as noted byboth Hougen 41 and Brown, 42 should be most predominant inthe K   =1 levels and may be negligible in the other observed K   levels. A more detailed description of the energy levelstructure along with energy level diagrams for 2  E  and 2  A 1 states can be found in Refs.5,31,and32 The allowed rotational transitions between the B ˜  2  E  and  X  ˜  2  A 1 states are determined by the selection rules for aperpendicular-type transition,  K  =±1 and   J  =0,±1. In-cluding parity dependence, 43 this results in six branchesper  K  subband within each spin-orbit component. Eachindividual branch is labeled by the notation  K    J  F  i  F   j   i =1,2;  j =1,2  , where  K  is represented by a lower case  p   K  =−1  or r    K  =+1  . 42 Because the rotation of theborohydrides about the symmetry axis exchanges the protonsof the BH 4− ligand, the effect of nuclear spin statistics mustalso be considered. In this case, this phenomenon gives riseto two nuclear spin states: ortho  K  =3  N  , where N  is an in- FIG. 2. High resolution spectra of the two spin-orbit components of the  B ˜  2  E  -  X  ˜  2  A 1 transition of SrBH 4 and CaBH 4 are plotted on a relative wave-number axis scale for comparison. The B ˜  2  E  3/2 -  X  ˜  2  A 1 spin-orbit componentof SrBH 4 is shown in the bottom left panel. This component has an expectedappearance similar to a Hund’s case  a  2  –Hund’s case  b  2  + transitionwith  1  B and  3  B spaced branches. The B ˜  2  E  1/2 -  X  ˜  2  A 1 spin-orbit compo-nent of SrBH 4 is shown in the top left panel. This component has a veryunique structure with no clear srcin, and all of the lines of the observedbranches appear to be spaced by  2  B . Neither component of the B ˜  2  E  -  X  ˜  2  A 1 transition of CaBH 4  right side  has the appearance of a perpendicular tran-sition. Each component is highly congested and lacks any clear srcin,which hampered the rotational assignment of the lines in these spectra. 164311-3Spectra of CaBH 4 and SrBH 4 J. Chem. Phys. 126 , 164311  2007  Downloaded 14 Aug 2007 to 129.97.47.146. Redistribution subject to AIP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp  teger  and para  K   3  N   . 43 Molecules in each of thesenuclear spin states behave like separate species and cannotrotationally cool to one another in the free jet expansion. Asa result, both K   =0 and K   =1 levels are populated in the jet.Therefore, according to the selection rule  K  =±1, three  K  subbands should be predominantly observed in our spectra, K   =1 ← K   =0, K   =0 ← K   =1, and K   =2 ← K   =1, whichgive rise to 36 branches in total.Rotational assignments of the B ˜  2  E  -  X  ˜  2  A 1 transition of SrBH 4 were completed by first examining the B ˜  2  E  3/2 -  X  ˜  2  A 1 spin-orbit component. The similar appearance of this compo-nent with the A ˜  2  E  3/2 -  X  ˜  2  A 1 transition of SrCH 35 facilitatedbranch assignments. Because no previous ground state com-bination differences were available, rotational assignmentswere made within each  K  subband using first lines and thengenerated lower state combination differences. Subsequently,this set of combination differences was used to make rota-tional assignments in the B ˜  2  E  1/2 -  X  ˜  2  A 1 component, whichlacked any similarity with the corresponding component of SrCH 3 . In this case, a series of branches on each side of thesrcin were first identified and then grouped together utiliz-ing the combination differences. Using this method, 17 of the18 possible branches from all three observed  K  subbandswere identified in this spin-orbit component. Figure3showsa portion of the spectra with line assignments   K    J  F  i  F   j   i =1,2;  j =1,2  , for each spin-orbit component of the  B ˜  2  E  -  X  ˜  2  A 1 transition of SrBH 4 . The unusual structure of the  B ˜  2  E  1/2 -  X  ˜  2  A 1 component, as compared to the B ˜  2  E  3/2 -  X  ˜  2  A 1 spin-orbit component, can clearly be observed in the fourbranches  r  P 11 , r  Q 12 , p  R 12 , and p Q 11  that have convergednear the srcin and have adopted an appearance similar to astrong Q branch.The assignment of the B ˜  2  E  -  X  ˜  2  A 1 transition of CaBH 4 was not as straightforward. Unfortunately, neither spin-orbitcomponent exhibited the expected appearance of a perpen-dicular transition, and both were exceptionally congested. Inaddition, each component lacked a clear srcin, making itdifficult to identify the starting position of any branch. As a FIG. 3. Subsections of the high resolution spectra of the B ˜  2  E  1/2 -  X  ˜  2  A 1  toppanel  and the B ˜  2  E  3/2 -  X  ˜  2  A 1  bottom panel  spin-orbit components of SrBH 4 . For each component, 15 of the 18 possible branches are shown. Ineach spectrum, branches assigned to the K   =1 ← K   =0 subband are labeledon the bottom in italics and those belonging to the K   =2 ← K   =1 and K   =0 ← K   =1 subbands are labeled on the top. Individual rotational linesare labeled by the notation  K    J  F  i  F   j   i =1,2;  j =1,2  . In the B ˜  2  E  1/2 -  X  ˜  2  A 1 spin-orbit component  top panel  the rotational lines of 11 of the 15 ob-served branches are spaced by  2  B and the r  P 11 , r  Q 12 , p  R 12 , and p Q 11 branches have almost converged near the srcin.FIG. 4. A portion of the high resolution spectra, including rotational assign-ments, for each spin-orbit component of the B ˜  2  E  1/2 -  X  ˜  2  A 1 transition of CaBH 4 . Again, the K   =1 ← K   =0  on the bottom in italics  , K   =2 ← K   =1, and K   =0 ← K   =1  on the top  subbands are shown, and individualrotational lines are labeled by  K    J  F  i  F   j   i =1,2;  j =1,2  . For the  B ˜  2  E  3/2 -  X  ˜  2  A 1 spin-orbit component  bottom panel  , the p Q 22 and p P 21 branches begin at the blue of the r  Q 21 and r   R 22 branches, resulting in a veryoverlapped and congested structure for this spin-orbit component. The  B ˜  2  E  1/2 -  X  ˜  2  A 1 spin-orbit component  top panel  has a clear srcin; however,the p P 11 branch can only be ambiguously assigned, suggesting a perturba-tion  see text  . 164311-4 Dick et al. J. Chem. Phys. 126 , 164311  2007  Downloaded 14 Aug 2007 to 129.97.47.146. Redistribution subject to AIP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp
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