Rotationally Resolved REMPI Spectra of CaH in a Molecular Beam

Journal of Molecular Spectroscopy 212, 17–21 (2002) doi:10.1006/jmsp.2002.8531, available online at http://www.idealibrary.com on

Rotationally Resolved REMPI Spectra of CaH in a Molecular Beam R. Pereira,∗ S. Skowronek,∗ A. Gonz´alez Ure˜na,∗,1 A. Pardo,† J. M. L. Poyato,† and A. H. Pardo† ∗ Unidad de L´aseres y Haces Moleculares, Instituto Pluridisciplinar, Universidad Complutense de Madrid, Juan XXIII-1, 28040-Madrid, Spain; and †Departamento de Qu´ımica F´ısica Aplicada, Facultad de Ciencias, Universidad Aut´onoma de Madrid, Cantoblanco 28049-Madrid, Spain E-mail: [email protected] Received May 30, 2001; in revised form January 15, 2002

Rotationally resolved spectra of the CaH radical using a supersonic molecular beam are reported. Thus, high energy resolution of the (1 + 1 ) REMPI spectra corresponding to the A ← X and B ← X electronic transitions were measured which allowed for the first time a clear and precise analysis of the low rotational part (J ≤ 7) of the CaH spectrum. A comparison with previous studies revealed that the commonly accepted energy separation between the two band heads of B − X (v  = 1, v  = 0) of 10 cm−1 is not correct. A value of ca. 1.6 cm−1 was found to be more realistic. On the other hand, the present study confirmed that the C 2002 Elsevier Science (USA) perturbation appears only for v  = 1, N  > 3 of the B state.

exception of the works of Weinstein et al., only one rotational level). Obviously, at these temperatures high J -value rotational lines could be observed with no intensity problems but lines with low J -values were difficult to measure due to their much weaker intensities. Therefore, in spite of all the above-mentioned studies, little, if any, experimental information is available on the low-J part of the CaH rotational spectrum. This situation motivated the present work in which a rotationally cold CaH beam was produced by Ca laser vaporization and subsequent supersonic expansion using H2 /He mixtures as carrier. This method allowed for the observation of CaH rotational lines up to J ≈ 7 in both A–X and B–X transitions with appreciable intensity and very high spectral resolution. As a result, the direct observation, study, and interpretation of this part of the CaH spectrum have been possible for first the time since the early days of Hulth´en nearly a century ago.

I. INTRODUCTION

The first rotational analysis of the CaH band spectra of the A 2 –X 2  + and B 2  + –X 2  + transitions were made by Hulth´en (1) in 1927. Later, Liberale and Weniger (2) recorded CaH absorption spectra in a graphite resistance furnace. More recently, Berg and Klynning (3) measured the absorption spectrum of CaH in the region 613.0–712.5 nm and reported wavenumbers of many rotational lines belonging to the A 2 –X 2  + and B 2  + –X 2  + transition. Using these data, the same authors (3) reinvestigated both transitions and interpreted the results with the use of simple theoretical models. One of the most relevant conclusions of such study was the finding of a perturbation in the P2 branch of the (1,1) band of the B 2  + –X 2  + transition for low J -values. In 1976 Berg et al. (4) studied the B 2  + states of CaH using laser excitation spectroscopy with a cw dye laser. Both (1,0) and (2,1) bands of the B 2  + –X 2  + transitions were measured. For the (1,0) band, they found an energy separation of 10 cm−1 between the two bandheads of the B–X transition. More recently, Martin (5, 6) elucidated extensively about the double minimum of the B 2  + state. He found that a perturbation appears for v  = 1, N  = 3. The latest studies of CaH electronic states are the works of Weinstein et al. (7, 8), who investigated the CaH molecule using a millikelvin magnetic trap. An important conclusion of these works is the existence of an inverted multiplet structure for the B 2  + state. A similar conclusion is obtained in the present work, but here it is proposed for the X 2  + state. A common feature of all these experimental investigations was the presence of a considerable number of rotational lines as well as several vibrational bands due to the high temperature of the CaH produced in a furnace, typically at ca. 1000◦ C (with the 1

II. EXPERIMENTAL

A detailed description of the experimental set-up employed in this work has been given elsewhere (9, 10): atomic Ca is produced by laser vaporization of a rotating Ca disk, using the second harmonic output of a Nd-YAG laser with an energy of 1 mJ. The CaH molecules are formed in the subsequent supersonic expansion of the Ca vapour in a gas pulse of a mixture of He and H2 (∼20%). The total backing pressure is 2 bar. The adiabatic expansion is skimmed about 40 mm downstream from the vaporization source by a 2 mm skimmer and the molecular beam passes into the detection chamber. CaH ions are produced by REMPI technique and detected in a 1-m linear TOF spectrometer using a dual MCP detector. The MCP output is registered in a Le Croy LC334AM digital oscilloscope, connected to a PC.

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A tuneable dye laser with 0.08 cm−1 bandwidth is used to produce the CaH excited state, while the excited molecules are ionized by the 4th harmonic of a Nd-YAG laser. The 2-ns laser pulses arrive at about the same time at the beam–laser interaction region. The ionization laser energy is adjusted to about 260 µJ, such that no CaH+ signal is observed with the ionization laser alone. The excitation laser energy, which is a function of the wavelength, is registered together with the CaH signal during the experiment. The spectrum of a Sn hollow cathode lamp filled with Ne is used to calibrate the excitation laser wavelength. III. RESULTS AND DATA ANALYSIS

Figure 1 shows the spectrum of the CaH molecule in the range 14 330–14 570 cm−1 . This spectrum displays the rotational lines of the (0,0) band of the A 2 −X 2  + electronic transition. Figure 2 shows the spectrum of the CaH molecule in the range 16 940–17 070 cm−1 , which corresponds to the rotational structure of the (1,0) band of the B 2  + –X 2  + electronic transition. These assignments are consistent with the spectroscopic constants of the CaH molecule (11) and the rotational temperature of the spectrum. Due to the nature of the electronic states involved in these transitions and assuming a Hund’s case “b” for the 2  states and a Hund’s case “a” for the 2  state we can expect twelve branches for the A–X system and six branches for the B–X system. However, from the twelve branches only eight are observed for the A–X system due to the small multiplet splitting in the X state. These branches are called R, Q, and P depending on the J = +1, 0, −1, respectively and can also be differentiated by the F1 and F2 components. Thus, we have Q 1 , P12 , R1 , Q 12 , P1 , and R12 for the 2 1/2 term and P2 , Q 2 , R21 , Q 21 , P2 , and

FIG. 1. REMPI spectrum of the CaH molecule in the range 14 330– 14 570 cm−1 , obtained by exciting the molecule with a tuneable dye laser and ionising the excited molecule with the 4th harmonic of a Nd-YAG. The rotational lines of the (0,0) band of the A 2 –X 2  + electronic transition can be observed. Each branch has been marked with a different symbol, as indicated.

FIG. 2. Same as Fig. 1, but now for the range 16 940–17 070 cm−1 , corresponding to the (1,0) band of the B 2  + –X 2  + electronic transition. Each branch has been marked with a different symbol, as indicated.

R2 for the 2 3/2 one. All these branches are clearly seen in the spectrum except for the ones that overlap. Moreover, the reduced number of lines for each of the branches is consistent with the low temperature of the spectrum. A multiplet splitting of ∼70 cm−1 is also seen, as well as a very small  splitting of 1.5 cm−1 for the 1/2 and even smaller for the 3/2 component. The branches corresponding to this transition for N = ±1 and J = 0, ±1 are P1 , P2 , R1 , R2 , P Q 21 , and R Q 12 . In the B state there is a strong perturbation for N  > 3 that affects the J  = 5/2 component (this is represented by a very low line intensity and a shift of the energy level which falls underneath the J  = 7/2). This occurs for all components with N  > 3, as can be noticed in Fig. 3, where the different branches

FIG. 3. Lower part: CaH rotational spectrum for the (1,0) band of the B 2  + –X 2  + electronic transition (same as Fig. 1). Upper part: N  values for the same transition. The values that lie above and below the dashed line correspond to the perturbed components. Notice the onset of the perturbation for N  > 2 (i.e. N  > 3) marked with an arrow.

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ants can be considered. Only the electronic energies are presented in Tables 1 and 2 with the variance of the fit. The medium calculated X Bv=0 rotational constant (4.20966 cm−1 ) is in good agreement with that obtained in previous works. The A Bv=1 rotational constants for the multiplet components f f 2 e 1/2 , 2 1/2 , 2 3/2 and 2 e3/2 , are 3.988, 4.098, 4.4690, and 4.5356 cm−1 , respectively. As mentioned before, the CaH spectra in the red-region are well known and have been extensively studied by many groups (1–8). For example, the pioneering work of Hulth´en (1) is one of the first spectroscopic studies of CaH. Looking at the spectral analysis part of all these works, it becomes clear that one of the mayor difficulties in the characterisation and assignment of the spectral lines of the excited states A and B and the ground FIG. 4. Spin doubling for the R and P branches for v  = 1 of the B-state as a function of N  .

are observed. The upper part of the figure also displays the N  values with the nonperturbed components linked by a straight line. These effects are better seen for the R1 and R2 branches because the P1 and P2 overlap with lines from some other transition. Concretely, the perturbed branches are R1 and P1 , that is, the perturbed terms are the F1 ones of the B-state for N  > 3. Figure 4 shows the spin splitting for the R and P branches as a function of N  . The same feature was presented by Martin in Fig. 7 of Ref. (5). Comparing both figures (Fig. 4 of the present work and Fig. 7 from Ref. (5)), it can be seen that in both cases the perturbation starts at N  > 3, but the spin splitting is different. This difference could be due to an incorrect assignment of the first lines of the R1 and P1 branches. The main features of the rotational structure for these type of transitions are well described in the literature. The reader is addressed to the textbook of Herzberg (Ref. (12)) for detailed information. Clearly the stronger lines are associated with the higher statistical weights which implies larger J  with lower energies. For high N  there is a perturbation and the energies of the J  components are inverted while the J  components keep their behaviour. Tables 1 and 2 list the assigned lines for the A–X and B–X transitions, respectively, together with the quantum numbers and the fit spectroscopic electronic energies. In the A–X transition only a splitting in the A state for the four components f f 2 e 1/2 , 2 1/2 , 2 3/2 , and 2 e3/2 , has been considered. For the B–X transition we have considered only the splitting in the B state. No splitting in the X state is considered because this is very small compared to the resolution of the experimental data; see Petitprez et al. (13). As can be noticed from the statistical parameters there is just one component with a strong perturbation for the B–X transition (Table 2, R1 , Q 12 , and P1 , Te = 16 999.2 var. = 1.02), but all others behave reasonably well, although the number of lines is small and not many const-

TABLE 1 Assignment of Spectral Lines for the (0,0) Band of the A2 Π−X 2 Σ+ Electronic Transition. Both Quantum Numbers and Fitted Electronic Energies are Shown. The Variances of the Fittings are Also Indicated A− X J

N 

J 

ν (cm−1 )

J

N 

J 

R 12 3/2 5/2 7/2 9/2 11/2 13/2

0 1 2 3 4 5

1/2 3/2 5/2 7/2 9/2 11/2 Q1

1/2 3/2 5/2 7/2 9/2 11/2

f 1/2 Te

14 408.94 14 421.12 14 432.92 14 444.54 14 455.76 14 467.20

3/2 5/2 7/2 9/2 11/2 13/2

0 1 2 3 4 5

3/2 5/2 7/2 9/2 11/2 13/2

Q 21 14 387.66 14 383.61 14 378.96 14 374.12 14 370.09 14 363.43

3/2 5/2 7/2 9/2

2 3 4 5

14 480.93 14 495.08 14 510.09 14 525.53 14 541.43 14 557.98

P2

3/2 5/2 5/2 7/2 7/2 9/2 9/2 11/2

14 455.76 14 453.10 14 451.06 14 449.83

e3/2 Te = 14 467.3 cm−1 var. = 0.006

Q 12

0 1 1/2 2 3/2 3 5/2 4 7/2 5 9/2 6 11/2

1/2 3/2 5/2 7/2 9/2 11/2 13/2

R21 14 395.36 14 398.60 14 401.84 14 404.88 14 407.72 14 410.56 14 413.40

3/2 5/2 7/2 9/2 11/2 13/2 15/2

2 3 4 5

3/2 5/2 7/2 9/2

14 472.53 14 478.06 14 484.20 14 490.76 14 497.75 14 505.15 14 512.56

P2 14 370.09 14 356.57 14 342.88 14 329.03

e1/2 Te = 14 395.2 cm−1 var. = 0.02

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Q2

1 1/2 3/2 2 3/2 5/2 3 5/2 7/2 4 7/2 9/2 5 9/2 11/2 6 11/2 13/2 7 13/2 15/2

P1 1/2 3/2 5/2 7/2

1/2 3/2 5/2 7/2 9/2 11/2

= 14 396.7 cm−1 var. = 0.013 R1

1/2 3/2 5/2 7/2 9/2 11/2 13/2

R2

P12

1 1/2 2 3/2 3 5/2 4 7/2 5 9/2 6 11/2

ν (cm−1 )

3/2 5/2

3 4

5/2 7/2

14 430.48 14 v419.09

3/2 Te = 14 467.5 cm−1 var. = 0.002 f

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TABLE 2 Assignment of Spectral Lines for the (1,0) Band of the B 2 Σ+ –X 2 Σ+ Electronic Transition. The Quantum Numbers and the Electronic Energies are Shown B–X J N

J 

N  ν (cm−1 )

J N

R2 3/2 5/2 7/2 9/2 11/2 13/2 15/2

1/2 3/2 5/2 7/2 9/2 11/2

1 2 3 4 5 6 7

0 1 2 3 4 5

1/2 3/2 5/2 7/2 9/2 11/2 13/2 P2

Q 21

5/2 7/2 9/2 11/2 13/2

1/2 3/2 5/2 7/2 9/2 11/2

0 1 2 3 4 5 6

17 005.97 17 014.39 17 022.95 17 031.63 17 040.68 17 049.72 17 059.02

1/2 3/2 5/2 7/2 9/2 11/2 13/2

1 2 3 4 5 6 7

J  R1

Q 12

1/2 3/2 5/2 7/2 9/2 11/2

1/2 3/2 5/2 7/2 9/2 11/2 13/2

N  ν (cm−1 )

0 1 2 3 4 5 6

17 007.14 17 015.80 17 020.96 17 027.99 17v036.92 17 046.44 17 056.08

1 2 3 4 5 6

16 989.59 16 981.98 16 973.69 16 962.00 16 951.87 16 943.48

P1 1 2 3 4 5 6

16 989.59 16 980.70 16 972.17 16 963.88 16 955.83 16 948.41

1/2 1/2 3/2 5/2 7/2 9/2

0 1 2 3 4 5

3/2 3/2 5/2 7/2 9/2 11/2

the proximity of both electronic states A and B, which are revealed by the special behavior shown by the spectral lines at certain progressions. These difficulties have fairly been overcome in the two transitions investigated in the present work: only the lower-lying rotational states are significantly populated because of the low rotational temperature of the molecular beam (about 40 K). Thus, the measured spectra are delimited, and could be analysed practically without ambiguity. As seen before, the identification of the branches is in complete agreement with the predictions for the rotational structure of the 2  + –2  + and 2 –2  + transitions. The perturbations were clearly detected by analysing the intensities, and the statistic parameters indicate whether the behavior of the branches is regular or not. Some examples will show how the present study compares to previous works:

state X , resides in the proximity of these two excited states. This complexity is due to the fact that both transitions occur in the same spectral region, and that both states arise from the same atomic configuration. Also, apart from the difference in electronic energy, they have almost identical electronic potentials in the equilibrium zone (see theoretical studies of CaH, Ref. (14, 15)). Furthermore, strong perturbations arise due to

a. For the A–X transition: Berg and Klynning (3, 4) indicate that they analyzed the band heads P1 , P2 , Q 2 , and Q P12 for the transition A–X (v  = 0, v  = 0). In the present work, the corresponding study has been carried out for 8 branches (4 of the 12 possible branches are overlapped). Also, Fig. 5 displays the branches assigned by Hulth´en (1) for the A–X (v  = 0, v  = 0) transition, and those analyzed in the present work. b. Concerning the B–X transition: Fig. 6 shows the branches assigned by Berg, Klynning, and Martin for the B–X (v  = 1, v  = 0) transition, which can be compared with the results of the present work, shown in Fig. 3. One of the branches P and R match with the ones observed in this work, but the other P and R branches are obviously not correctly assigned. The same happens with the work of Hulth´en. This indicates that the assumption made about the existence of two bandheads separated

FIG. 5. R, Q, and P branches assigned to the observed rotational lines for the (0,0) band of the A2 –X 2  + electronic transition. Large points connected by dashed lines: present work; small points: branches assigned by Hulth´en (1).

FIG. 6. Branches assigned by Berg, Klynning and Martin from their data on rotational lines (4) for the B 2  + – X 2  + (v  = 1, v  = 0) transition (compare with our data shown in Fig. 3).

Te = 16 997.6 cm−1

var. = 0.05

Te = 16 999.2 cm−1

var. = 1.02

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by about 10 cm−1 is not correct. Our analysis clearly indicates that the value of ca. 1.6 cm−1 for the bandhead separation is more realistic. The structure of this transition is the one that is shown in the Figs. 2 and 3.

ACKNOWLEDGMENTS Financial support from the DGICYT of Spain (Projects PB96-0046 and PB970272) is gratefully acknowledged. The Complutense University group also acknowledges the financial support by the Fundaci´on Ram´on Areces.

REFERENCES

IV. CONCLUSIONS

This paper has been dedicated to the measurements of the rotationally resolved REMPI spectra of the CaH radical using a cold molecular beam. This condition produced a significant population of the lower-lying rotational states of the ground electronic state. Furthermore, the high energy resolution of the laser (a linewidth of 0.08 cm−1 ) employed in the first electronic A ← X or B ← X transitions of the (1 + 1 ) REMPI process allowed for the first time a clear and precise analysis of the low rotational part of the CaH spectrum. Indeed, the identification of the branches is in complete agreement wit the expected rotational structure for the 2  + –2  + and 2 –2  + transitions. A complete analysis was carried out for eight branches and a comparison was made with previous analysis made by Hulth´en for the A ← X transition and by Berg et al. for the B ← X transition. Perhaps one of the most relevant conclusions of the present work is that the commonly accepted energy separation between the two band heads of B–X (v  = 1, v  = 0) band of 10 cm−1 is not correct. Our analysis clearly indicates that the value of ca. 1.6 cm−1 for the bandhead separation is more realistic. On the other hand, the present work confirms that only for v  = 1N  > 3 in the B state a perturbation appears, as originally reported by Martin (5).

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