Chemical Physics Letters 591 (2014) 43–46

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Chemical Physics Letters

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Jet-cooled laser-induced dispersed fluorescence spectroscopy of NiC:Observation of low-lying X = 0+ state

0009-2614/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.cplett.2013.10.085

⇑ Corresponding author at: Atomic and Molecular Physics Division, BhabhaAtomic Research Centre, Mumbai 400 085, India. Fax: +91 22 25505151.

E-mail address: nakhate@barc.gov.in (S.G. Nakhate).

Sheo Mukund a,b, Suresh Yarlagadda b, Soumen Bhattacharyya a, S.G. Nakhate a,b,⇑a Atomic and Molecular Physics Division, Bhabha Atomic Research Centre, Mumbai 400 085, Indiab Homi Bhabha National Institute, Bhabha Atomic Research Centre, Mumbai 400 085, India

a r t i c l e i n f o

Article history:Available online 11 November 2013

a b s t r a c t

Laser-induced dispersed fluorescence spectra of 58Ni12C molecules, produced in a free-jet apparatus, havebeen studied. A new low-lying X = 0+ state has been observed at Te = 5178 (6) cm�1. Based on previousab initio calculations this state is plausibly assigned as 0+ spin–orbit component of the first excited 3Pstate. The term energies of vibrational levels up to v = 10 for X1Rþ ground and v = 3 for X = 0+ states havebeen determined. The harmonic and anharmonic wavenumbers respectively equal to 833 (4) and 6.7(13) cm�1 for X = 0+ state have been measured.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

The transition metal carbides are of considerable current inter-est due to their importance in catalysis, organometallic chemistry,and biological processes. The diatomic transition metal carbidesprovide simple model systems for understanding the bonding ofa carbon atom to transition metals and helps in understandinglarger transition-metal-containing systems like organometallics.The experimental spectroscopic parameters also provide rigorousbenchmarks for theoretical calculations of electronic structure insystems with open d-subshells that result in large numbers oflow-lying states [1]. Transition metal carbides, particularly thoseof the 3d period, are also of astrophysical importance and are likelyto be detected in circumstellar environments of carbon-rich stars[2,3]. However, among transition metal carbides, gas phase spec-troscopic investigations in the 3d period are meager and only VC[4], CrC [5], FeC [6–12], CoC [13,14], and NiC [3,15] have beenstudied. In this Letter, we present our results of spectroscopicinvestigations on NiC molecule.

The first spectroscopic observation of NiC diatomic in the gasphase was reported by Brewster and Ziurys [3] in the millimeterregion, providing the ground state (X1Rþ) rotational constants tohigh accuracy. Later Brugh and Morse [15] studied the jet-cooledNiC molecule by R2PI in the optical region and observed numeroustransitions originating from ground X1Rþ state to excited elec-tronic states, all having X = 0+. In contrast to experimental studies,NiC has been theoretically studied extensively. The first, ratherprimitive, theoretical calculations was reported in 1982 by Kitauraet al. [16] using the generalized valence bond approach. They

calculated the bond length, r0 = 1.80 Å and dissociation energy,De = 23 kcal mol�1, deviated strongly from the experimental values[15]. A few years later, Shim and Gingerich [17] calculated low-ly-ing singlet, triplet and quintet states of Rþ, P, D, and U symme-tries, including the 1Rþ ground state, derived from 3d84s1, 4F Ni+

and a 2s22p3, 4S C� ionic states based on all electron ab initio calcu-lations. The first excited state was found to be a 1P state at1532 cm�1. The NiC bond was found to be quite polar, with sub-stantial electron transfer from the Ni atom to the C ligand. Shimand Gingerich later revisited the NiC molecule at the completeactive space self-consistent-field (CASSCF) level and found thatthe ground state X1Rþ is now separated from a dense manifoldof excited states, with the first excited 1P state lying at6465 cm�1 [18]. In 2001, Borin [19] examined the X1Rþ and A1Pstates of NiC using the multi-reference configuration interaction(MRCI) approach based on the CASSCF wavefunctions and reportedpotential energy curves, spectroscopic constants, radiativelifetimes and dipole moments of the A1P-X1Rþ transition. Usingexactly the same approach, a few years later Borin and Macedo[20] calculated six more low-lying states and revised the energyordering of the lowest-lying singlet and triplet electronic states.They found 3P to be the lowest state, in contrast to the 1P statecalculated by Shim and Gingerich [18]. Recently, Tzeli and Mavridis[21] investigated the electronic structure of ground and low-lyingexcited states through multiconfiguration self-consistent field(MCSCF) calculations coupled with large basis sets and with inclu-sion of core-valence correlation and scalar relativistic effects. Theyfound that the X1Rþ, a3P and A1P states are well separated fromrest of the states and a3P is indeed the first excited state. Theexperimental observation of the energy ordering and character ofthe low-energy states can provide a validation of the level of the-oretical calculations. However, to the best of our knowledge,experimental detection of the low-lying states in NiC molecule

44 S. Mukund et al. / Chemical Physics Letters 591 (2014) 43–46

has not been reported. The confusion among theoretical investiga-tions over the energy ordering of the a3P and A1P states motivatedus to undertake dispersed fluorescence studies on diatomic NiC.

2. Experiment

The NiC molecules were produced in a pulsed free-jet appara-tus, details of which are discussed in [22]. A rotating and translat-ing nickel metal rod was ablated by �10 mJ/pulse, 355 nmradiation from a Q-switched Nd:YAG laser focused to an �1 mm2

spot. The resulting nickel plasma reacted with 3% methane seededin He emerging from a pulsed valve under a backing pressure of275 kPa. Approximately 5 cm downstream of the nozzle, the free-jet was probed by a tunable dye laser at right angles to the beamexpansion axis. The resulting laser-induced fluorescence (LIF)was collected orthogonal to both free-jet and dye laser beam axes,dispersed by a monochromator and subsequently detected by aPeltier cooled photomultiplier tube. An output signal from thephotomultiplier was amplified by a 1 GHz bandwidth amplifier,integrated by a gated integrator, and stored on a computer. Thedispersed fluorescence (DF) spectra were obtained from theexcited electronic states reported by Brugh and Morse [15] byexciting isolated rotational lines. Typical DF spectral line widths(FWHM) were �5 cm�1 at the best obtained resolution.

3. Observations and analysis

The DF spectra of NiC molecule were investigated in the 400–900 nm wavelength range by exciting a single rotational levelbelonging to 58Ni12C isotopomer of the several excited states withX = 0+ observed by Brugh and Morse [15]. The DF spectra were firstrecorded with moderate resolution (FWHM � 50 cm�1) to getcoarse information about the low-energy and ground states andtheir vibrational levels. The coarse spectra were then examinedunder relatively higher resolution (FWHM � 5 cm�1) to resolvethe rotational structure in the lower states for determining theidentity of the observed states.

3.1. Vibrational constants of the X1Rþ ground state

Figure 1 shows a typical DF spectrum of 58Ni12C isotopomer,where the P(8) line of the [24.6]0+-X1Rþ (0,0) transition [15] wasexcited. The vibrational progression from v00 = 0–10 of the X1Rþ

ground state was observed and is marked in Figure 1. The DF lines

0 2000 4000 6000 8000

X1Σ+

Displacement(cm−1)

a3Π0+

ν=0 1 2 3 4 5 6 7 8 9 10

ν=0 1 2 3

5x

R(6)P(8)

a3Π 0+ ν=1

Figure 1. Dispersed fluorescence (DF) spectra of 58Ni12C observed from m0 = 0, J0 = 7of [24.6] X = 0+ state; the x-axis shows the displacement in cm�1 from theexcitation line. The inset shows the DF spectra at high-resolution where the P(8)and R(6) lines are well resolved.

terminating to the ground state split into PR-doublets at higherresolution when either P- or R-lines of the excitation band were ex-cited. Since the upper state has X = 0 and the transitions are typicalof DX = 0, the lower levels were confirmed as the vibrations of theground electronic state. The term energies of the vibrational levelsof X1Rþ state are listed in Table 1. The term energies of the v00 = 1and 2 observed in the present Letter are in good agreement withthe tabulated values reported by Brugh and Morse [15] derivedfrom band origin data. The equilibrium vibrational constantsxe = 874.6 (17) cm�1 and xexe=5.9 (2) cm�1 of the X1Rþ state weredetermined by fitting the observed vibrational term energies to theequation:

DG ¼ GðmÞ � Gð0Þ ¼ xem�xexemðmþ 1Þ ð1Þ

3.2. Observation of low-lying X = 0+ excited electronic state

In addition to the vibrational progression to the ground state,the DF spectrum shows a vibrational progression beginning at5157 cm�1 as marked in Figure 1. Theoretical calculations predictlow-lying states 3P, 3Rþ, and 1P, in the energy region 6000–8000 cm�1 [18,20,21]. However, the energy ordering between thea3P and A1P states is in question. Shim and Gingerich [18] predictA1P as the first excited state 6465 cm�1 above the ground state,however recent calculations of Borin and Macedo [20] and Tzeliand Mavridis [21] predict a3P as the first excited state at�8000 cm�1. In order to confirm the identity of the low-lying stateobserved in the present Letter, the DF spectral line was studied athigher resolution to probe the rotational structure. The rotationalstructure of line terminating to the progression beginning at5157 cm�1 split into PR-doublets, when either P- or R-lines of the(0,0) band of the [24.6]0+-X1Rþ transition were excited (see insetof Figure 1). This is in accordance with DX = 0 transition, confirm-ing X = 0+ of the lower electronic state. This is the lowest excitedstate observed in the present work. The separation between thePR-doublets was used to estimate the rotational constant of theX = 0+ state and was found to be 0.59 (3) cm�1. The evaluated B va-lue is accurate only to the first decimal place and no meaningfulvalues for D could be determined. The ae value also could not bedetermined from the set of evaluated Bv

’s due to the low precisionof the DF data, which was limited by the resolution of the mono-chromator. The term energies of up to v = 3 were used to evaluatethe equilibrium vibrational constants xe = 833 (4) cm�1, xexe = 6.7(13) cm�1 and the equilibrium term energy, Te = 5178 (6) cm�1 ofthe X = 0+ state. The Be value of 0.593 (28) cm�1 and thusre = 1.69 (4) Å was estimated iteratively using the Pekeris relation-ship [23] from the rotational constant B = 0.59 (3) cm�1.

Table 1Term values (in cm�1) of the vibrational levels of the X1Pþ and X = 0+ states of58Ni12C.

m Tv X = 0+

X1P+

This work Ref. [15]

0 0 0.0 5157 (5)1 864 (3) 864.3919 5976 (9)2 1720 (5) 1718.0208 6783 (14)3 2555 (7) 7574 (14)4 3388 (8)5 4206 (3)6 4978 (3)7 5770 (7)8 6557 (6)9 7324 (7)

10 8101 (10)

Table 2Spectroscopic constantsa of the experimentally observed X1Pþ and X = 0+ states of58Ni12C and their comparison with ab initio values.

State Te re xe xexe

X1Pþ Present Letter 0.0 – 874.6 (17)b 5.9 (2)

Ref. [15]c 0.0 1.62729(17) 875.155 5.382Ref. [18]d 0.0 1.655 809 –Ref. [18]e 0.0 1.621 874 –Ref. [20]f 0.0 1.627 937 –Ref.[21]g 0.0 1.621 916.1 –

X = 0+ Present Letter 5178 (6) 1.69(4) 833 (4) 6.7 (13)3P Ref. [18]d 7451 1.957 564 –

Ref. [20]f 7882 1.620 937 –

S. Mukund et al. / Chemical Physics Letters 591 (2014) 43–46 45

In addition to the members of the two main DF progressions wehave also observed few weak fluorescence features mainly at 9371,9876, 10588, and 11342 cm�1 which may correspond to fluores-cence terminating on vibrational levels of the 1P, 3P or the 3Rþ

states, but definite assignments were not possible.The RKR potential energy curves for the X1Rþ and X = 0+ states

assigned as 3P0þ (see Section 4) were constructed using the RKR1program of LeRoy [24] and are shown in Figure 2. The vibrationalconstants from the present work and rotational constants (Be andae) from the earlier work [15] for X1Rþ and vibrational and rota-tional constants for 3P0þ from this work were used to generatethe RKR potential energy curves.

Ref.[21]g 8746 1.606 9811P Ref. [18]d 6465 1.942 524 –

Ref. [20]f 9427 1.773 539 –Ref.[21]g 12419 1.622 940 –

3Pþ Ref. [18]d 6772 1.960 557 –Ref. [20]f 10835 1.843 561 –Ref.[21]h 13504 1.669 874 –

a re in Å and other constants in cm�1.

b Numbers in parentheses indicate one standard deviation in the last quoteddigits.

c Experimental.d CASSCF including relativistic corrections.e MRCI.f MRCI.g C-MRCI+DKH2/C5Z.h MRCI.

4. Discussion

In the present work, we have observed a vibration progressionup to v = 10 for the ground state X1Rþ and evaluated vibrationalequilibrium constants xe and xexe which are in good agreementwith the values in Ref. [15], evaluated from the band origin dataup to v = 2 (see Table 2). In addition to this, we have observed avibrational progression up to v = 3 terminating on a new elec-tronic state at Te = 5178 (6) cm�1 with X = 0+, re = 1.69 (4) Å andxe = 833 (4) cm�1. The relevant electronic states and their spec-troscopic constants calculated by the ab initio studies[18,20,21], listed in Table 2, can be used to assign the newly ob-served electronic state. As can be seen from Table 2, Shim andGingerich [18] predicted 1P, 3Rþ and 3P states respectively at6465, 6772 and 7451 cm�1 using CASSCF calculations. However,on the basis of MRCI calculations, Borin and Macedo [20] placedthe 3P state energetically lower at 7882 cm�1 compared to the1P and 3Rþ states respectively at 9427 and 10 835 cm�1. RecentMCSCF/CISD calculations of Tzeli and Mavridis [21] had alsoshown that the 3P state is energetically lower at 8746 cm�1 com-pared to the 1P and 3Rþ states respectively at 12 419 and13 504 cm�1. It is known that the CASSCF method alone is not suf-ficient to describe the multitude and correct energy ordering ofthe excited states in transition metal carbides. The inclusion ofboth nondynamical and dynamical correlation effects are veryimportant in describing the excited states.

The computational work has shown that the lowest lying ex-cited states of NiC are of 3P, 3Rþ, and 1P symmetry. When spin–or-bit interaction is considered, these split into substates with X = 0+,0-, 1, and 2 from 3P; X = 0-, 1 from 3Rþ; and X = 1 only from 1Pstate. Because X = 0+

M X = 0� transitions are rigorously forbidden

1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.10

2000

4000

6000

8000

a3Π 0+

Ener

gy (c

m-1)

Internuclear distance (Å)

X1Σ+

Figure 2. RKR potential energy curves for the X1Pþ , and a3P0þ states of 58Ni12C.Observed vibrational levels are shown by solid lines.

under electric dipole selection rules, the X = 0+ state that is initiallyexcited can only radiate to states of X = 0+ or X = 1 symmetry. Theobservation of pairs of P, R lines demonstrates that the new state isof X = 0+ symmetry. If we accept the computational result that thelow-lying states are 3P, 3Rþ, and 1P, then the only possibility forthe newly-discovered low-lying state is 3P0þ , which is the lowestexcited state observed so far. However, it is likely that otherspin–orbit component of the 3P state will lie lower in energy. Ifthe 3P state is regular, resulting from 9r13p44p11d4 configuration[18], it should have spin–orbit levels in the order(X = 0+,0�) < X = 1 < X = 2. However, in that case the X = 0+ com-ponent of 3P will interact with the 1Rþ ground state by off-diago-nal spin–orbit coupling, causing it to be pushed above the X = 0�

component. This effect was observed in a dispersed fluorescencestudy of PdC [25]. On the other hand, if the 3P state is irregular,resulting from configuration 9r23p44p11d3 [18], the spin–orbitlevels will lie in the reversed order: X = 2 < X = 1 < (X = 0+,0�). Ineither case, there will be an excited X-level lying below the 3P0þ

state. The observation of emissions to the 1Rþ ground and 3P0þ

states and not to the other 3PX levels also suggests that the 3P0þ

state borrows much of its intensity in these emissions from the1Pþ ground state.

The comparison of observed spectroscopic constants for 3P0þ

state with theory shows that the energy order of 3P with respectto the 1P and 3Rþ states, its internuclear distance and vibrationalenergy predicted by the CASSCF and MRCI calculations differ sig-nificantly, with MRCI values closer to the observation (see Table 2).The present work supports the MRCI prediction of 3P to be the firstexcited state; however the term energy is overestimated by about40%. The CASSCF calculations overestimated re by �16% and under-estimated xe by �48%. However, MRCI values for re are about 4–5%lower and xe values are higher by about 10–15% than the ob-served. This is also valid for the ground state. Shim and Gingerich[18] calculated spectroscopic constants for X1Rþ ground state ofNiC, both at the CASSCF and MRCI level. As evident from Table 2,re and xe values predicted by them at MRCI level are closer tothe observed ones.

46 S. Mukund et al. / Chemical Physics Letters 591 (2014) 43–46

5. Conclusion

Laser-induced dispersed fluorescence spectra from isolatedrotational levels of the excited electronic states of 58Ni12C [15], leadto the observation of an excited electronic state plausibly assignedto 3P0þ with the support of earlier ab initio studies. The rotationallyresolved vibrational progression up to v = 3 yielded equilibriumconstants xe = 833 (4) and xexe = 6.7 (13) and Be = 0.593(28) cm�1 for this state. Also, the extended vibrational progressionup to v = 10 to the ground state lead to the determination ofxe = 874.6 (17) and xexe = 5.9 (2) cm�1 for the X1Rþ state. TheRKR potential energy curves for the X1Rþ ground and 3P0þ stateswere constructed.

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