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Jet emission spectra in the Schumann region: The L2Σ− → A2Πi transition of CN
JOURNAL OF MOLECULAR
SPECTROSCOPY
1%%,407-419
(1987)
Jet Emission Spectra in the Schumann Region: The !_*I?:-+ A211i Transition of CN K. P. HUBER, C. A. KLUG,’ AND F. ALBERTI Herzberg Institute of Astrophysics, National Research Council of Canada, Ottawa, Ontario KIA 0R6, Canada The emission spectrum of the CN radical in the ultraviolet and Schumann regions from approximately 3500 to 1500 A has been photographed in higher orders of a 10.6-m concave grating vacuum spectrograph. Of the two sources used to generate the spectra one is a radiofrequency discharge in flowing pure HCN, the other a dc discharge in supersonically expanding Ar seeded with HCN. An extensive system of red-degraded double-headed bands appears in the jet emission spectrum between 2000 and 1500 6;. The bands have a rotational temperature of -40 K and arise in transitions from eight vibrational levels of a new %- electronic state, labeled L*Z-, to the well-known A211iupper state of the CN red system. Vibrational and rotational constants have been determined, the new state has w, = 1165.80 and B, = 1.3337 cm-‘. Possible reasons for the lack of emission from 0 = 2 of L2z- are discussed. @ 1987 Academic RW, IIK. I. INTRODUCTION
The CN radical is an important contributor to the emission spectra of a wide variety of discharges, flames, and afterglows. It appears prominently in cometary spectra and, in absorption, in many stellar and interstellar sources. From the infrared through the visible to the near ultraviolet the spectrum of CN has been the subject of numerous publications which have contributed to a very detailed description of the three lowest electronic states, i.e., of X2Zf, A*&, and B*Z+,the latter two being the upper states of the so-called red and violet systems of CN, A + X and B + X,respectively. Much less attention has been focused on the study of the higher-lying valence states of CN. Douglas and Routly (I) investigated the emission from a discharge through helium mixed with traces of C2N2. Their analyses of rotationally resolved spectra in the 2000- to 3000-A region established the existence of a regular *A state, designated F2A,, at 59 68 1 cm-’ above 2, = 0 of the ground state and of a number of 211vibronic levels at energies ranging from 53 955 to 60 939 cm-’ and attributed to two, possibly three, new electronic states (@II,, H*II,, . . .). Carroll (2) extended the search into the Schumann region of the spectrum where he identified an upper state progression of bands originating from an inverted *A state-he took the lowest observed level at 64 785 cm-’ to be o = 0 of J2Aj-as well as several transitions from a previously unknown Z state, E*F. The latter is identical with the upper state of a weak absorption spectrum of ground state CN which Lutz (3) observed in flash discharges through mixtures of Ar + HCN; the u” = 0 progression extends from the O-O band at 58 960 ’ NRCC Summer Student 1983/84. 407
0022-2852187 $3.00 Copyrisht8 1987 by Academic FIBS, Inc. All righis of reproduction in any form reserved.
408
HUBER,
KLUG,
AND ALBERT1
cm-’ to the 5-O transition at 67 088 cm-‘. Two additional papers by Jha and Rao (4) and Lutz (5) relate to the F2A, state and complete the picture of the known doublet states of CN. The molecular constants have been reviewed in Huber and Herzberg (6). Still rather rudimentary is the information on low-lying quartet states of CN. Apart from ab initio calculations (7, 8), the evidence comes exclusively from the analysis of weak perturbations in the B21;+ state [see Ito et al. (9, IO) and references therein]. No transitions between quartet states have been reported so far, although two such states, the a4Z+ state near 37 000 cm-’ and the b411state at about 45 000 cm-‘, are sufficiently well characterized for the region of transitions between them to be predicted with some confidence. In the course of a search for emission spectra of HNC we found that a radiofrequency (rf) discharge through flowing pure HCN produced a band spectrum on either side of 2000 A that could be entirely ascribed to the CN radical. Particularly gratifying is the nearly complete absence of all strong CO emissions, but the spectra are characterized by fairly high rotational temperatures and, consequently, the analyses are hampered by the mutual overlapping of many of the bands. It turns out, however, that an equally pure and rotationally much colder spectrum can be obtained by replacing the rfdischarge in slowly flowing pure HCN by a dc discharge in supersonically expanding argon containing a trace of HCN. This jet emission technique, originally explored by Droege and Engelking (II), has been successfully applied in this laboratory to the study of complex spectra of ND4 (12) and NO (12, 13) as well as of ArXe+ (14). Its present extension from the infrared and visible into the Schumann region, down to about 1500 A, is particularly promising in view of the notoriously poor wavenumber resolution attainable in the vacuum ultraviolet (VUV), even with large scale instruments, which renders the analysis of dense and heavily overlapped spectra a difficult and often impossible task. The new emission spectra add in more than one way to our knowledge of the higher excited states of CN. They provide evidence for a new electronic state of ‘Z- symmetry; they lead to a new interpretation of the ‘II levels which differs from that given by Douglas and Routly (I) or Lutz (3), but is in line with results of ab initio calculations (7, 15, 16); and they show that the vibrational levels of J2Aj need to be renumbered, the observations of Carroll (2) having been extended to both higher and lower vibrational quantum numbers. In this first report we give an account of the experimental procedures and we describe the results of our analyses of emission bands that arise in transitions from the new 2Z- state, designated L2Z-, to A211i. II. EXPERIMENTAL
DETAILS
Figure 1 illustrates the two experimental arrangements that have been used to generate the CN emission spectra. For excitation by an electrodeless rf discharge (27 MHz) the saturated vapor from above a sample of liquid HCN was slowly pumped through the foreslit (S2) into the adjoining discharge tube of approximately 1 cm diameter and was then collected in a liquid-nitrogen-cooled trap (T). For the duration of the experiment the carefully purified sample of HCN, produced in the reaction of KCN with H2SO4 and normally stored at dry ice temperature, was immersed into a
THE L2Z- +
A211i TRANSITION
OF CN
409
FIG. 1. Schematic outline of the radiofrequency and jet discharge experiments. L,, quartz or lithium fluoride prism/cylinder combination acting as predisperser; L2, lithium fluoride sphericallens;S,, spectrograph slit; S2, foreslit; T, liquid-nitrogen-cooled trap.
water/ice mixture which maintained a constant reservoir pressure of about 260 Torr. The actual pressure in the discharge region was not measured, but the flow rate was monitored and was adjusted for maximum brightness in the 0- 1 sequence of the CN violet system at 42 16 A. Under these conditions the red system of CN was strongly developed, and the only visually detectable impurity was the strong A --f X O-Oband of CH at 43 14 A. During operation a black glassy deposit formed on the walls of the discharge tube, eventually blocking the flow of HCN and making it necessary to clean or replace the Pyrex tube from time to time. For the jet emission experiments Ar seeded with HCN was expanded from a highpressure reservoir through a nozzle of approximately 0.3 mm diameter into the lowpressure region from where it was removed by an EDWARDS EH5OO/E2M80 pump combination having a capacity of 140 liters/set. With a reservoir pressure of one atmosphere or more this pumping speed was adequate for maintaining a pressure of a few hundred mTorr in the expansion chamber. The latter was made of Plexiglas and was sitting directly on top of the booster pump. The quartz nozzle has been described before (11, 12). A voltage of +500 V applied to the tungsten anode inside the quartz tube is sufficient to maintain a dc discharge through the expanding jet, with the pump assembly acting as cathode. However, the anode and the nozzle walls soon become coated by the same glassy material that was already noticed in the rf experiment, causing frequent interruptions of the discharge. In order to obtain a more reliable operation during long exposure times it was necessary to increase the voltage to +700 V, with discharge currents of typically 80 mA. The light emitted from the rf or jet discharge and passing through the foreslit (S,) was collected by a lithium fluoride spherical lens (Lz) and subsequently predispersed and refocused onto the slit of the spectrograph (S,) by a prism/cylinder combination (L,) made of lithium fluoride or quartz, depending on the wavelength region examined. The pump + nozzle assembly like the table carrying the rf discharge tube could be rotated around a pivot directly underneath the predisperser (L,), making it possible to select and isolate a narrow wavelength region for analysis in higher orders of the
410
HUBER,
KLUG,
AND
ALBERT1
10.6-m concave grating vacuum spectrograph. Of the two gratings used, one, blazed at 6000 A and having 1200 lines/mm, served for work in second or third order at wavelengths longer than 2000 A, the other, blazed at 11 600 A and having 600 lines/ mm, was chosen for work in sixth or seventh order at shorter wavelengths. In spite of the generally good performance of the order separator the sixth- and seventh-order jet spectra were not entirely free from overlapping third-order spectra of the strongest bands in the CN violet system. We blame this difficulty primarily on the overwhelming strength of the violet emission which, with the proper setting of the spectrograph, is readily photographed in as little as 10 set, while the CN spectra recorded in the Schumann region are relatively weak, requiring exposure times of up to one or several hours. Atomic lines of carbon and nitrogen appear with only moderate intensities in the rf discharge, but they give rise to very strong emissions in the jet spectra where the groups of carbon lines near 1657 and 1561 A tend to interfere with the measurements of the much weaker molecular bands. Most of the spectra were photographed on Kodak SWR plates, some of the weakest features on Kodak 101-05 special plates. They were measured against iron lines in overlapping orders. III. THE
L*B- --fA'IIj SYSTEM OF CN
In most aspects the VUV emissions from the jet discharge correspond well with spectra generated by the more conventional rf discharge. The jet spectra exhibit, however, some noticeable differences which go beyond the effects caused by rotational cooling of the emitting species in the supersonic expansion. The strongest features in both types of discharges are due to transitions from the F2Ar and J2Ai states to A211i, but several of the other vibronic levels which give rise to comparatively strong emissions in the rf discharge fail to emit from the jet source. The E’Z+, v = 0 level shows little or no evidence of rotational cooling. Most significant is the appearance in the jet spectrum of a new extensive system of red-degraded and double-headed bands. In the rf discharge many of these bands are hidden underneath the strongly developed rotational structure of other transitions, and their detection from a high-temperature source is made even more difficult by the shifting away of the intensity from the bandheads, which will be seen to be formed by lines of very low rotational quantum numbers. A good example is provided by the spectra reproduced in Figure 2. The spectrum of the rf discharge in Fig. 2a is almost entirely due to high Jlines of the J2Ai --, A211i2-l band which has its head at 53 883 cm-‘, far off the left-hand margin of the figure. By contrast, in the jet spectrum of Fig. 2b the high J lines of the J + A transition are virtually absent, making way for the appearance of a double-headed structure that belongs to the O-2 transition of the new band system. Table I shows the new bands to lie on a wide open Condon parabola. The R, 1 and Q22heads listed in the table-anticipating the results of the analysis-are particularly evident on first-order medium-resolution plates. With increased dispersion and resolution the higher members in the u” = 0 progression are seen to be so strongly degraded that first in the RI, branch, then also in &, a true head can, in fact, no longer be formed. The lower state vibrational intervals are in perfect agreement with those of A*&. The smaller intervals in the upper state are reminiscent of frequencies
THE L*Z- +
a
1861.893
A211i TRANSITION
411
OF CN 1865.733
FIG. 2. The CN emission spectrum between 53 580 and 53 730 cm-‘. (a) Radiofrequency discharge in pure HCN, showing high J lines in the 2-l band of J2Ai + A211,(sixth order, 600 lines/mm grating). (b) Jet discharge in Ar + HCN, showing the O-2 band of L2Z- + A211i(third order, 1200 lines/mm grating).
in F*A, and J2Ai, suggesting a close connection with these two states. Indeed, the rotational analysis that follows identifies the new electronic state as 22- which, in all probability, is one of four Z states belonging to the same electron configuration . . . U~TT~C~T that gives rise to the two 2A states. The new electronic state will be designated L2T. Its lowest observed level is assigned to tl = 0 without additional supporting evidence. The rotational analysis is straightforward and poses no problems. Two of the bands are shown in Figs. 2 and 3. They have the typical structure of a case (b) --, case (a) *L: + 211 transition where the upper state spin splitting is too small to be resolved, while the lower state splitting is large and is reflected by the 56-cm-’ interval which separates the two subbands. Each subband comprises a maximum of four branches, none of them showing signs of A doubling in accordance with the Z + II character of the transition. By and large, the strongest lines belong to the 4 form branches which result from the superposition of a main with a satellite branch. Of the $ form branches only PII is well developed, the head forming RZI and R22 branches are generally weak, and fragments of a Pi2 branch in the 1-O band are somewhat uncertain. The line measurements and rotational assignments are collected in Table II; the J numbering was confirmed with the help of known combination differences for the A211i state. The table includes only measurements of jet emission spectra. In order to establish the + or - symmetry of the upper state we have compared the combination defect
412
HUBER, KLUG, AND ALBERT1 TABLE I Wavenumbers (cm-‘) and Wavelengths (A) of the RI, and Q12Heads in the L2Z- --t A211iSystem of ‘??N 1
““ZO
RII Qzz
“‘C
0
v 55471.7 u 55415.5 X 1802.72 X 1804.55
2 1762.1
5370g.6 53653.5 1861.86 1863.81
AG 1141.1 1
58400.4 58344.1
1787.6
;;;;;‘; 1766:39
1712.32 1713.97
3
3 1736.5
51973.1 51917.0 1924.07 1926.15
1141 .o 1762.1
;;;;t’; 1823: 13 1825.00
1768.14
60607.8 60551.6 1649.95 1651.48 1066.9
4
61674.7 61618.4 1621.41 1622.89
Comparison intervals
with AG(v+%) vibrational of CN from other sources:
1041.9 5
6
”
A2il
0
1787.71
1214.01
1147.26
1
1762.14
1188.52
1120.99
1018.0
2
1736.56
63734.4 63678.6 1569.01 1570.39
3
1067.28
4
1041.32
5
1016.8
62716.6 62660.4 1594.47 1595.90
i
F2Ar
J2Ai
1093.94
992.7 7
64727.2 64671.2 1544.95 1546.28
Ref.
(18)
(5)
(2), this
ad work
968.5 8
65695.6 65639.8 1522.17 1523.47
Q,,(J)-PI,(J+ with the sum of the known
A
I)-&I(J)+QII(J+
1)
(1)
splittings for the lower state terms J and J + 1, Av,dJ) + Av,AJ+
1).
The labels e and f have their standard meanings (I 7), and the sign of the A splitting is defined by Av~~J) = F’(J) - Fe(J). (3) Expressions (1) and (2) are easily verified to be equal for a Z- upper state, but to have opposite signs if the upper state is F. The experimental evidence derived from the rf
THE L22- +
I 61700 B
I
I
I
1
A*l& TRANSITION
I
I
I
61600 1622.562
413
OF CN
I
I
I 61500
t
I
I 1626.694
C" -I
FIG. 3. The 4-O band of L*Z- + A*Q in emission from the jet discharge. The cold spectrum with T,,,t PZ 40 K is overlapped by a weaker emission of considerably higher temperature.
excited spectrum of the O-2 transition is shown in Fig. 4. Within experimental uncertainties the combination defects (l), represented by open circles, are consistent with the curve corresponding to expression (2), the latter evaluated from the L~*II~,~term values of Cerny et al. (28). The sign of the combination defect clearly points to a Zupper state. Addition of the observed line frequencies to the A*& term values (18) results in upper state energies that reduce to the vibronic energies and rotational constants summarized in Table III. Where the range of observed rotational levels was too limited for a reliable determination of higher-order constants in the least-squares fit, the centrifugal correction D was kept fixed at the calculated value 4Bj/w%. The spectra of the jet discharge have a rotational temperature of approximately 40 K as estimated from an underexposed plate of the 4-O band. In longer exposures like that of Fig. 3 the cold spectrum is usually overlapped by a weaker but rotationally much hotter spectrum which quite possibly originates in the shock regions surrounding the expanding jet. This presents no problems for the analysis; on the contrary, in a few instances it enabled us to extend the measurements to sufficiently high J numbers for the error limits on D to be substantially narrowed. IV. DISCUSSION
The presently available information on the doublet states of configuration . . . is reviewed in Table IV where the equilibrium constants from experimental data are compared with the corresponding ab initio calculated parameters of Schaefer and Heil (7). Although the agreement is only moderately good, the deviations of the calculated from the experimental results are seen to behave in fairly systematic fashion, and it is evident that the new t*I;- state must be associated with the second root of this symmetry obtained by Schaefer and Heil(7), the first root corresponding to a still undetected ‘Z- electronic state at around 60 500 cm-‘. The close correspondence &r3mr
10 11
:
J”-%
%1
51973.12 973.12 972.44 971.19 969.39 967.00 964.03 960.45 956.43
53709.61 709.61 708.80 707.43 705.42 702.83 699.60 695.79 691.26 686.14 680.40 673.92
5544;;:;;
55471.68 471.68 470.76
o-2
437.89 429.81 421.00
5 5463.75 458.41 451.99
o-1
pll
51967.87 965.16 961.83 957.97 953.54 948.48 942.86 936.66 929.86 922.42 914.39 905.93 896.66 aab.73 876.31
51965.16 959.86 953.94 947.37 940.23 932.58 924.30 915.27 905.93 896.04
“-1
_-
53701.66 53704.43 696.35 701.66 698.19 690.32 694. ia 683.56 676.29 689.56 668.41 684.31 659.92 678.47 650.84 671.97 664.82 657.11 648.70 639.67
460.08 455.94 451.16 445.60 439.57 432.90 425.41 417.25 408 52
Q11+p21
%1*Q21
642.36 637.75 632.39 626.28 619.49 611.70 603.30 59L.16 584.28 573.67
66::::: 658.85
928.49 928.95 928.95 927.85 926.25 923.84
646.18 649.19
:66::88:
sIi927.1a
651.42 652.94
648.34 644.99 640.92 635.98 630.33 623.87 616.61 608.63 599.84 590.23 579.97 568.96
:i3650.84
55410.27 406.90 402.65 397.63 391.74 304.99 377.56 369.19
55415.52 415.52 414.84 413.20 410.83 407.59 403.50 398.76 392.88
53653.53 653.53
p22+Q12
Q22+R12
Z::
13658.85
1 661.58
R22
PI2
1 :
0
1:
ii
6’
: 3 4
0
9” 10 11
: 7
;
:
0
:: 12 13 14
2 6’ ; 9
:
0
J”-%
58408.27 410.51 412.10 412.61 412.61 412.10 410.51 408.27
R21
60607.82 607.24 605.68
56612.31 611.35
58400.04 398.93 397.03 394.39 391.09 387.01 382.21 376.76 370.53
*11+921
60602.65 599.64 595.65
796.08
:;9’:;: 572.49
56607.40 604.57 600.92 596.59 591.61
:45::;: 332.34
:%: 378.77 372.89 366.27 358.88
58395.22 392.23
Q11+P21
60600.25 594.61 588.03
3-o
54842.72 837.41 831.25 824.54 817.02 808.94 800.19 790.88
l-2
56599.31 593.14 585.97 578.51
l-l
327.32 315.85
58392.60 387.01 380.67 373.48 365.82 357.26 348.06
1-O
pll
344.65
350.96
58349.35 351.82 353.67 354.21 354.21
R22
Wavenumbers (cm-‘) and Rotational Assignments for Lines in the L*Z- + A211iBands of ‘*C14N
TABLE II
60551.59 551.59 550.28 548.05
54794.53 794.53 793 .a4 792.09 789.62 786.43 782.32 777.51 771.76 765.22 757.84
56556.46 556.46 555.64 553.86 551.29 547.82 543.47 538.29 532.19 525.28 517.57 508.94
ii
58344. 344.11 343.09 341.28 330.54 334.85 330.29 324.84 x8.51 311.23 303.15 294.29 284.58 273.97 262.38
Q22*R12
60549.15 546.44 542.72 537.93
54789.32 706.01 781.72 776.70 770.83 764.09 756.73 748.36
56553.86 551.29 547.82 543.47 538.29 532.19 525.28 517.57 508.94
:‘8: .:t 274156 262.98 250.72
58338.86 335.28 330.81 325.44 319.18 311 .a8
p22+Q12
58315.00 306.08 296.35
P12
i%:::
z:::: 684.76 683.90
61682.33
62716.64 715.76 113.94 711.44 707.83 703.46 698.14 691.97 685.07
61674.65 673.88 672.23 669.73 666.44 662.28 657.35 651 .57 644.96 637.50
565.12
60603.46 600.25 596.69 591.97 586.59 580 .oa
5-o
:
:;:.:: 557 64 542.64 526.46 509.78 491.96 473.31 453.89 433.49 412.15
61667.14 661.43 654.75 641.36 639.07 629.90 620.02 609.40 597.58
4-o
:45::;; 532.21 519.80 506.99
3-o (con1 60580.61 572.62 563.68
62711.68 62709.32 708.35 703.46 704.12 699.04 %: :! 693 09 680.63 686.24 671.54 678.55 661.34 669.92 650.15 660.39 638.38
507.68 575.16 561.41 547.10 531.87 515.88 498.71 480.78 461.93 442.21 421.67 400.20
61669.73 666.44 662.28 657.35 651.57 644.96 637.50 629.19 620.02 609.97
578.80 571.74 563.68 554.97 544.97 534.91 523.67
6”;;;:;;
Ix: 667.59 665.43
62665.43 667.59
:;XZ 583.29
612.66 606. a9
627.02 621.02 626.27 624.37 621.45
61623.57
60552.42 548.05 542.72 536.48
‘d)
62660.39 660.39 658.18 656.34 652.78 648.17 642.55 635.86 628.09 619.47
616.97 614.60 611.21 606. a9 601.53 595.13 587.68 579.42 570.01 559.75 540.42 536.05 522.05 508.67 493.51 477.38 460.20 442.21 423.20 403.23
6’;;;:;;
60544.97 540.73 535.81 529.60 522.55 514.61 505.76
62658.16 655.39 651.48 646.52 640.53 633.49 625.44 616.26 606.02 594.99
K: 584.11 575.16 565.38 554.54 542.64 529.93 515.88 501.30 485.61 468.86 451 .25 432.67 413.04
61616.15 613.24 609.48 604.64
489.39
60532.21 525.63 517.94 509.42
TABLE
i
9 10 11
1: 11 12 13
i
2
3 4
:
0
H-Continued
:Ki: 691.39
:2’:::: 716.90 711.93
64727.la 725.99
:::: :t 720.11 714.56 708.20
63734.35 133.36 731.32
682.71 677.09 670.61 663.12 654.59 645.08 634.49 622.83
65;;;:;:
714.29 708.99 702.71 695.29 687.08 617.89 667.70 656.72 644.44
64;;;::;
716150 710.31 703.30 695.32 686.44 676.51
63129.54 :x
62650.15 638.89
675.61 667.76 658.90
“‘;“,W:;;
a-o
707.17 699.37 690. a2 681.07
ti4;;94:;98
7-o
706.87 698.09 688.81 678.59
63727.03 721.25 714.56
6-O
5-O (cmt’d) 62625.44
65637.43 634.49 630.36 625.16 618.61 610.97 602.15 592 29 581.20
%::I: 576.54
64669.06 666.20 662.18 657.23 651.06 643.40 635.32 625.18 615.15
670.03 665.06 659.26 652.35 644.26 635.11
;;.:;;
63678.59
65637.43 634.49 630.36 625.16 618.61 610.97 602.15 592.29 581.20 568.86 555.56 541 .oo
64669.06 666.20 662.18 656.12 650.56 643.08 634.47 624.88 614.03 602.28 589.09 574.98
657.85 650.76 642.39 633.09 622.58
63;;::;;
416
HUBER, KLUG, AND ALBERT1 cm“ I
- 0.1
0 0 I,
I
IO
I
I
I,
I
20
I
I
J-h
FIG. 4. Combination defects (open circles) according to expression (1) for the L*X- + A*& O-2 transition of CN. The curve represents the lower state A splittings, summed according to expression (2) and evaluated from Ref. (18).
between vibrational and rotational constants for the 2A and 28- states underlines their derivation from a common electron configuration. The E22+ state, on the other hand, deviates considerably from the other four states in the table. This state is strongly mixed with B2Z+ (7); the effects of the perturbation are significantly increased o, and Be values and a strongly anharmonic potential curve. Based on the predicted proximity of the two 2Z- states one might expect the L -+ A system to be accompanied by a second band system of similar appearance, shifted to longer wavelengths but reaching with a long upper state progression well into the region of the L + A bands. This has not been observed; all strong features shortward of 2000 A have now been accounted for, and there is no evidence for emission from high vibrational levels of a second 22- state. This does not preclude, however, the possibility that this state emits at longer wavelengths, in the dense and still incompletely analyzed region from 2000 to 2500 A. A wider than anticipated spectral separation of the two systems could, e.g., come about if the electronic transition moments were strongly r dependent. In this context it is instructive to note that of the two 2A + 211systems the J + A bands [(2), and this work] are largely confined to wavelengths shorter than 2000 A and tend to arise from transitions between high vibrational levels in the initial and low vibrational levels in the final state. By contrast, the F + A bands (1, 4, 5) are prominent at longer wavelengths and combine low vibrational levels of the upper with high vibrational levels in the lower state; they are virtually absent from the shortwavelength region. The Deslandres tables of both systems have lopsided intensity distributions which emphasize only one or the other branch of the Condon parabola, the full parabola becoming apparent, however, when the two Deslandres tables are superposed on each other. Some of the gaps in the observations may, of course, disappear as the analysis progresses toward longer wavelengths. Nevertheless, the available evidence seems to point to marked variations in the electronic transition moments of J2A -+ A211 and F2A -WA211.The J + A system favors transitions at short internuclear
THE L*Z- +
A211i TRANSITION
417
OF CN
TABLE III Molecular Constants for the L*Z- State of ‘*(?‘N from Least-Squares Fits to the Upper State Term Values of the L + A ~‘-1)”Transitions Range of fitted rot. levels (N’)
1_“I1
v
106xD;
T’ ”
Order of spectrum
Discharge
ofit
o-1 o-2 o-2 o-3
l...lO 3...28 0 16 1:::15
66345.96(3) 66345.92(7) 66346.01(2) 66345.97(5)
1.3239(5) i.3238(5) 1.3236(5) 1.3245(11)
7.0 6.8(6) 5.i(ia) 9.1(47)
0.019 0.054 0.016 0.029
Jet R.F. Jet Jet
1-o 1-o l-l l-2
5...29 0...15 O...ll l...ll
67486.93(11) 67486.95(6) 67487.01(6) 67487.10(5)
1.3051(7) 1.3053(5) 1.3037(10) i .3045(a)
7.6(8) 7.0 7.0 7.0
0.066 0.047 0.046 0.035
R.F. Jet Jet Jet
3-o
0...12
69694.59(6)
1.2662(8)
7.0
0.048
Jet
6
4-o 4-o
O...lO 0...23
70761.53(4) 70761.51(4)
1.2457(a) 1.2464(5)
7.0 6.7(9)
0.029 0.037
Jet Jet
7 6
5-o
O...ll
71803.59(6)
1.2279(9)
7.0
0.042
Jet
7
6-O
O...lO
72821.41(6)
i.2085(10)
7.0
0.041
Jet
7
3 3 66 :
7-o
1...14
73814.36(a)
1.1878(a)
7.0
0.061
Jet
7
a-o
O...ll
74782.90(3)
i.l689(5)
7.0
0.022
Jet
7
Note:
-
T(, relative
-
Uncertainties place
and
fixed
at
to
X2E+(v=0, (in
correspond the
N=O).
parentheses) to
calculated
30.
are
in
units
DG values
value
of
without
the error
last
quoted
limits
decimal
were
kept
De=4Be3/we2.
TABLE IV Doublet Electronic States of Configuration . . . (rZr30r.Experimental and Ab Initio Calculated Equilibrium Constants for ‘*C14N State
w e
Te
wx e e
a
*e
Ref.
r,(d)
e
L2E-
talc. obs.
70200 66797.25
1183.2 1165.80
14.04 12.41aa
1.142 1.33367
0.0157 0.01937
1.468 1.3986
(7) This
work
J2A,b 1
talc. obs.
68010 62963.15
1211.7 1174.a9
17.58 13.72’
1.159 1.34706
0.0159 0.02063
1.457 1.3916
(7) This
workd
2z-
talc. obs.
64430
1225.5
14.92
1.165
0.0155
1.453
(7)
F2A e r
talc. ohs.
63550 60095.64
1253.7 1239.50
15.35 12.75
1.152 1.3834
0.0138 0.0187
1.461 1.3732
(7) (l)(5)
E22+
talc. obs.
63360 59151.1a
1717.1 16a1.43
30.57f 3.60
1.322 i .487i
0.0151 0.00649f
1.364 1.3245
(7) (2)(3)
ab’Y
= +o.oo76. dee preliminar12re;;lts. firmed
eA v
by
= +29.18
C/ -
bAv = -28.27 Vibrational C isotope o.as(v+*).
+ 0.594(V+k). numbering
of
‘WeYe = +0.048. Ref.(2) increased
shifts. fweye
= -1.02;
Ye = -0.00077
(v<4).
by +2 and
con-
418
HUBER, KLUG, AND ALBERT1
distances over those taking place at increased r values; in the F + A system the trend is just reversed. The detailed analysis of the emission spectrum longward of 2000 A will show if a similar situation applies to the *Z- states. Unfortunately, it appears that no ab initio calculations of the relevant transition moments have been carried out so far. The observed *2- vibronic levels are remarkably free of perturbations. However, no emission from v = 2 of L*Z-has been seen. In particular, the 2-O band is absent, and this in spite of the fact that both 1-O and 3-O appear with measurable intensity on our plates. The L*Z-,2)= 2 level can be calculated to lie at 68 603 cm-r above X*2+, v = 0 and to coincide almost exactly with the extrapolated position of E*2+, v = 6 at -68 617 cm-‘. Lutz (3) searched for the latter in the absorption spectrum, but, although he observed the E + X d-0 progression up to the relatively strong 50 band, he was unable to make a definitive identification of the 6-O band. The only evidence for absorption near the expected position of 6-O came from a low-dispersion plate where Lutz noticed a diffuse band with a rotational structure not unlike that of the E + X bands. This feature failed, however, to show up under high dispersion. Because of the very close approach of E*2+,o = 6 to L*Z-,o = 2 it is tempting to argue that the observed anomalies in the emission and absorption spectra are the result of an interaction between these two levels. The necessary matrix element is provided by the microscopic form of the spin-orbit interaction operator, ECaili- Si. A perturbation by *X- could readily account for a small shift and weakening of the E*Z+ + X22+ 6-O band. Conversely, the same interaction might be sufficient to wipe out all emission from 2) = 2 of L*Z-if the higher vibrational levels of E*Z+,particularly 21= 6, were subject to weak predissociation. For 2)< 5 they appear with perfectly sharp rotational structure in the absorption spectrum, but it may be significant that no emission has ever been seen from E2Zf levels higher than 2) = 1, the v = 2 level at 62 285 cm-’ being just within the error limits of the best currently available value for the dissociation energy of CN, Dt = 62 500 +- 800 cm-‘. The latter follows from thermochemical data in Refs. (19) and (20). It is obvious that the identification of the LLZ- + ALIIi system in the emission spectrum of CN would have been extremely difficult and time consuming if it had had to rely exclusively on spectra generated by the rf or some other standard type discharge. The comprehensive analysis presented here is proof of the considerable potential of the jet emission technique in the vacuum ultraviolet region as well as in the visible or infrared. For use in the Schumann region the small dimensions of the source may, on occasion, create alignment problems, and the relative weakness of the emission from this source runs counter to the constant need for more intensity in the vacuum ultraviolet, but these disadvantages are more than outweighed by the remarkable spectral purity of the source and by rotational temperatures which-though not spectacularly low by today’s standards-are just about ideal for the performance of a rotational analysis. ACKNOWLEDGMENTS We thank Dr. G. Herzbexg and Dr. J. K. G. Watson for reading this manuscript and for their helpful comments. RECEIVED:
February 2, 1987
THE L*Z- +
&Ii
TRANSITION
OF CN
419
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.
20.
A. E. D~LJGLAS AND P. M. ROUTLY,Astrophys. J. Suppl. Ser. 1,295-317 (1955). P. K. CARROLL,Can&. J. Phys. 34,83-95 (1956). B. L. LUTZ, C’unad. J. Phys. 48, 1192-1199 (1970). B. L. JHAAND D. R. RAo, Proc. Indian Acad. Sci. A 63, 3 16-320 (1966). B. L. LUTZ, Astrophys. J. 164,213-216 (1971). K. P. HUBER ANDG. HERZBERG, “MolecularSpectra and MolecularStructure,IV. Constantsof Diatomic Molecules,” Van Nostrand-Reinhold, New York, NY, 1979. H. F. SCHAEFER AND T. G. HEIL, J. Chem. Phys. 54,2573-2580 (1971). P. J. BRUNA,H. D~HMANN,AND S. D. PEYERIMHOFF, Canad. J. Phys. 62, 1508-1523 (1984). H. ITO,Y. OZAKI,T. NAGATA,T. KONDOW,ANDK. KUCHITSU,Cunod. J. Phys. 62,1586-l 598 (I 984). H. ITO, Y. OZAKI, T. NAGATA,T. KONDOW,K. KUCHITSU,K. TAKATSUKA,H. NAKAMURA,AND Y. OSAMURA,Chem. Phys. 98,8 l-87 (1985). A. T. DROEGEAND P. C. ENGELKING, Chem. Phys. Lett. 96,316-318 (1983); P. C. ENGELKING, Rev. Sci. Instrum. 57, 2274-2277 (1986). K. P. HUBERANDT. J. SEARS,Chem. Phys. Lett. 113,129-134 (1985). K. P. HUBER,M. VERVLOET,CH. JUNGEN,AND A. L. ROCHE,Mol. Phys. (1987), in press. K. P. HUBERAND R. H. LIPSON,J. Mol. Spectrosc. 119,433-445 (1986). G. DAS, T. JANIS,AND A. C. WAHL, J. Chem. Phys. 61, 1274-1279 (1974). H. LAVENDY,G. GANDARA,AND J. M. ROBBE,J. Mol. Spectrosc. 106, 395-410 (1984). J. M. BROWN,J. T. HOUGEN,K. P. HUBER,J. W. C. JOHNS,1. KOPP,H. LEFEBVRE-BRION, A. J. MERER,D. A. RAMSAY,J. ROSTAS,AND R. N. ZARE,J. Mol. Spectrosc. 55,500-503 (1975). D. CERNY,R. BACIS,G. GUELACHVILI, AND F. Roux, J. Mol. Spectrosc. 73, 154-167 (1978). D. D. WAGMAN, W. H. EVANS,V. B. PARKER,R. H. SCHUMM,I. HALOW,S. BAILEY,K. L. CHURNEY, AND R. L. NU~ALL, The NBS Tables of Chemical Thermodynamical Properties, J. Phys. Chem. Ref Data 11, Suppl. 2 (1982). M. W. CHASE,J. L. CURNUTT,J. R. D~WNEY, R. A. MCDONALD,A. N. SYVERUD,AND E. A. VALENZUELA,JANAF ThermochemicalTables, 1982 Supplement,J. Phys. Chem. Ref: Data 11,695940 (1982).
SPECTROSCOPY
1%%,407-419
(1987)
Jet Emission Spectra in the Schumann Region: The !_*I?:-+ A211i Transition of CN K. P. HUBER, C. A. KLUG,’ AND F. ALBERTI Herzberg Institute of Astrophysics, National Research Council of Canada, Ottawa, Ontario KIA 0R6, Canada The emission spectrum of the CN radical in the ultraviolet and Schumann regions from approximately 3500 to 1500 A has been photographed in higher orders of a 10.6-m concave grating vacuum spectrograph. Of the two sources used to generate the spectra one is a radiofrequency discharge in flowing pure HCN, the other a dc discharge in supersonically expanding Ar seeded with HCN. An extensive system of red-degraded double-headed bands appears in the jet emission spectrum between 2000 and 1500 6;. The bands have a rotational temperature of -40 K and arise in transitions from eight vibrational levels of a new %- electronic state, labeled L*Z-, to the well-known A211iupper state of the CN red system. Vibrational and rotational constants have been determined, the new state has w, = 1165.80 and B, = 1.3337 cm-‘. Possible reasons for the lack of emission from 0 = 2 of L2z- are discussed. @ 1987 Academic RW, IIK. I. INTRODUCTION
The CN radical is an important contributor to the emission spectra of a wide variety of discharges, flames, and afterglows. It appears prominently in cometary spectra and, in absorption, in many stellar and interstellar sources. From the infrared through the visible to the near ultraviolet the spectrum of CN has been the subject of numerous publications which have contributed to a very detailed description of the three lowest electronic states, i.e., of X2Zf, A*&, and B*Z+,the latter two being the upper states of the so-called red and violet systems of CN, A + X and B + X,respectively. Much less attention has been focused on the study of the higher-lying valence states of CN. Douglas and Routly (I) investigated the emission from a discharge through helium mixed with traces of C2N2. Their analyses of rotationally resolved spectra in the 2000- to 3000-A region established the existence of a regular *A state, designated F2A,, at 59 68 1 cm-’ above 2, = 0 of the ground state and of a number of 211vibronic levels at energies ranging from 53 955 to 60 939 cm-’ and attributed to two, possibly three, new electronic states (@II,, H*II,, . . .). Carroll (2) extended the search into the Schumann region of the spectrum where he identified an upper state progression of bands originating from an inverted *A state-he took the lowest observed level at 64 785 cm-’ to be o = 0 of J2Aj-as well as several transitions from a previously unknown Z state, E*F. The latter is identical with the upper state of a weak absorption spectrum of ground state CN which Lutz (3) observed in flash discharges through mixtures of Ar + HCN; the u” = 0 progression extends from the O-O band at 58 960 ’ NRCC Summer Student 1983/84. 407
0022-2852187 $3.00 Copyrisht8 1987 by Academic FIBS, Inc. All righis of reproduction in any form reserved.
408
HUBER,
KLUG,
AND ALBERT1
cm-’ to the 5-O transition at 67 088 cm-‘. Two additional papers by Jha and Rao (4) and Lutz (5) relate to the F2A, state and complete the picture of the known doublet states of CN. The molecular constants have been reviewed in Huber and Herzberg (6). Still rather rudimentary is the information on low-lying quartet states of CN. Apart from ab initio calculations (7, 8), the evidence comes exclusively from the analysis of weak perturbations in the B21;+ state [see Ito et al. (9, IO) and references therein]. No transitions between quartet states have been reported so far, although two such states, the a4Z+ state near 37 000 cm-’ and the b411state at about 45 000 cm-‘, are sufficiently well characterized for the region of transitions between them to be predicted with some confidence. In the course of a search for emission spectra of HNC we found that a radiofrequency (rf) discharge through flowing pure HCN produced a band spectrum on either side of 2000 A that could be entirely ascribed to the CN radical. Particularly gratifying is the nearly complete absence of all strong CO emissions, but the spectra are characterized by fairly high rotational temperatures and, consequently, the analyses are hampered by the mutual overlapping of many of the bands. It turns out, however, that an equally pure and rotationally much colder spectrum can be obtained by replacing the rfdischarge in slowly flowing pure HCN by a dc discharge in supersonically expanding argon containing a trace of HCN. This jet emission technique, originally explored by Droege and Engelking (II), has been successfully applied in this laboratory to the study of complex spectra of ND4 (12) and NO (12, 13) as well as of ArXe+ (14). Its present extension from the infrared and visible into the Schumann region, down to about 1500 A, is particularly promising in view of the notoriously poor wavenumber resolution attainable in the vacuum ultraviolet (VUV), even with large scale instruments, which renders the analysis of dense and heavily overlapped spectra a difficult and often impossible task. The new emission spectra add in more than one way to our knowledge of the higher excited states of CN. They provide evidence for a new electronic state of ‘Z- symmetry; they lead to a new interpretation of the ‘II levels which differs from that given by Douglas and Routly (I) or Lutz (3), but is in line with results of ab initio calculations (7, 15, 16); and they show that the vibrational levels of J2Aj need to be renumbered, the observations of Carroll (2) having been extended to both higher and lower vibrational quantum numbers. In this first report we give an account of the experimental procedures and we describe the results of our analyses of emission bands that arise in transitions from the new 2Z- state, designated L2Z-, to A211i. II. EXPERIMENTAL
DETAILS
Figure 1 illustrates the two experimental arrangements that have been used to generate the CN emission spectra. For excitation by an electrodeless rf discharge (27 MHz) the saturated vapor from above a sample of liquid HCN was slowly pumped through the foreslit (S2) into the adjoining discharge tube of approximately 1 cm diameter and was then collected in a liquid-nitrogen-cooled trap (T). For the duration of the experiment the carefully purified sample of HCN, produced in the reaction of KCN with H2SO4 and normally stored at dry ice temperature, was immersed into a
THE L2Z- +
A211i TRANSITION
OF CN
409
FIG. 1. Schematic outline of the radiofrequency and jet discharge experiments. L,, quartz or lithium fluoride prism/cylinder combination acting as predisperser; L2, lithium fluoride sphericallens;S,, spectrograph slit; S2, foreslit; T, liquid-nitrogen-cooled trap.
water/ice mixture which maintained a constant reservoir pressure of about 260 Torr. The actual pressure in the discharge region was not measured, but the flow rate was monitored and was adjusted for maximum brightness in the 0- 1 sequence of the CN violet system at 42 16 A. Under these conditions the red system of CN was strongly developed, and the only visually detectable impurity was the strong A --f X O-Oband of CH at 43 14 A. During operation a black glassy deposit formed on the walls of the discharge tube, eventually blocking the flow of HCN and making it necessary to clean or replace the Pyrex tube from time to time. For the jet emission experiments Ar seeded with HCN was expanded from a highpressure reservoir through a nozzle of approximately 0.3 mm diameter into the lowpressure region from where it was removed by an EDWARDS EH5OO/E2M80 pump combination having a capacity of 140 liters/set. With a reservoir pressure of one atmosphere or more this pumping speed was adequate for maintaining a pressure of a few hundred mTorr in the expansion chamber. The latter was made of Plexiglas and was sitting directly on top of the booster pump. The quartz nozzle has been described before (11, 12). A voltage of +500 V applied to the tungsten anode inside the quartz tube is sufficient to maintain a dc discharge through the expanding jet, with the pump assembly acting as cathode. However, the anode and the nozzle walls soon become coated by the same glassy material that was already noticed in the rf experiment, causing frequent interruptions of the discharge. In order to obtain a more reliable operation during long exposure times it was necessary to increase the voltage to +700 V, with discharge currents of typically 80 mA. The light emitted from the rf or jet discharge and passing through the foreslit (S,) was collected by a lithium fluoride spherical lens (Lz) and subsequently predispersed and refocused onto the slit of the spectrograph (S,) by a prism/cylinder combination (L,) made of lithium fluoride or quartz, depending on the wavelength region examined. The pump + nozzle assembly like the table carrying the rf discharge tube could be rotated around a pivot directly underneath the predisperser (L,), making it possible to select and isolate a narrow wavelength region for analysis in higher orders of the
410
HUBER,
KLUG,
AND
ALBERT1
10.6-m concave grating vacuum spectrograph. Of the two gratings used, one, blazed at 6000 A and having 1200 lines/mm, served for work in second or third order at wavelengths longer than 2000 A, the other, blazed at 11 600 A and having 600 lines/ mm, was chosen for work in sixth or seventh order at shorter wavelengths. In spite of the generally good performance of the order separator the sixth- and seventh-order jet spectra were not entirely free from overlapping third-order spectra of the strongest bands in the CN violet system. We blame this difficulty primarily on the overwhelming strength of the violet emission which, with the proper setting of the spectrograph, is readily photographed in as little as 10 set, while the CN spectra recorded in the Schumann region are relatively weak, requiring exposure times of up to one or several hours. Atomic lines of carbon and nitrogen appear with only moderate intensities in the rf discharge, but they give rise to very strong emissions in the jet spectra where the groups of carbon lines near 1657 and 1561 A tend to interfere with the measurements of the much weaker molecular bands. Most of the spectra were photographed on Kodak SWR plates, some of the weakest features on Kodak 101-05 special plates. They were measured against iron lines in overlapping orders. III. THE
L*B- --fA'IIj SYSTEM OF CN
In most aspects the VUV emissions from the jet discharge correspond well with spectra generated by the more conventional rf discharge. The jet spectra exhibit, however, some noticeable differences which go beyond the effects caused by rotational cooling of the emitting species in the supersonic expansion. The strongest features in both types of discharges are due to transitions from the F2Ar and J2Ai states to A211i, but several of the other vibronic levels which give rise to comparatively strong emissions in the rf discharge fail to emit from the jet source. The E’Z+, v = 0 level shows little or no evidence of rotational cooling. Most significant is the appearance in the jet spectrum of a new extensive system of red-degraded and double-headed bands. In the rf discharge many of these bands are hidden underneath the strongly developed rotational structure of other transitions, and their detection from a high-temperature source is made even more difficult by the shifting away of the intensity from the bandheads, which will be seen to be formed by lines of very low rotational quantum numbers. A good example is provided by the spectra reproduced in Figure 2. The spectrum of the rf discharge in Fig. 2a is almost entirely due to high Jlines of the J2Ai --, A211i2-l band which has its head at 53 883 cm-‘, far off the left-hand margin of the figure. By contrast, in the jet spectrum of Fig. 2b the high J lines of the J + A transition are virtually absent, making way for the appearance of a double-headed structure that belongs to the O-2 transition of the new band system. Table I shows the new bands to lie on a wide open Condon parabola. The R, 1 and Q22heads listed in the table-anticipating the results of the analysis-are particularly evident on first-order medium-resolution plates. With increased dispersion and resolution the higher members in the u” = 0 progression are seen to be so strongly degraded that first in the RI, branch, then also in &, a true head can, in fact, no longer be formed. The lower state vibrational intervals are in perfect agreement with those of A*&. The smaller intervals in the upper state are reminiscent of frequencies
THE L*Z- +
a
1861.893
A211i TRANSITION
411
OF CN 1865.733
FIG. 2. The CN emission spectrum between 53 580 and 53 730 cm-‘. (a) Radiofrequency discharge in pure HCN, showing high J lines in the 2-l band of J2Ai + A211,(sixth order, 600 lines/mm grating). (b) Jet discharge in Ar + HCN, showing the O-2 band of L2Z- + A211i(third order, 1200 lines/mm grating).
in F*A, and J2Ai, suggesting a close connection with these two states. Indeed, the rotational analysis that follows identifies the new electronic state as 22- which, in all probability, is one of four Z states belonging to the same electron configuration . . . U~TT~C~T that gives rise to the two 2A states. The new electronic state will be designated L2T. Its lowest observed level is assigned to tl = 0 without additional supporting evidence. The rotational analysis is straightforward and poses no problems. Two of the bands are shown in Figs. 2 and 3. They have the typical structure of a case (b) --, case (a) *L: + 211 transition where the upper state spin splitting is too small to be resolved, while the lower state splitting is large and is reflected by the 56-cm-’ interval which separates the two subbands. Each subband comprises a maximum of four branches, none of them showing signs of A doubling in accordance with the Z + II character of the transition. By and large, the strongest lines belong to the 4 form branches which result from the superposition of a main with a satellite branch. Of the $ form branches only PII is well developed, the head forming RZI and R22 branches are generally weak, and fragments of a Pi2 branch in the 1-O band are somewhat uncertain. The line measurements and rotational assignments are collected in Table II; the J numbering was confirmed with the help of known combination differences for the A211i state. The table includes only measurements of jet emission spectra. In order to establish the + or - symmetry of the upper state we have compared the combination defect
412
HUBER, KLUG, AND ALBERT1 TABLE I Wavenumbers (cm-‘) and Wavelengths (A) of the RI, and Q12Heads in the L2Z- --t A211iSystem of ‘??N 1
““ZO
RII Qzz
“‘C
0
v 55471.7 u 55415.5 X 1802.72 X 1804.55
2 1762.1
5370g.6 53653.5 1861.86 1863.81
AG 1141.1 1
58400.4 58344.1
1787.6
;;;;;‘; 1766:39
1712.32 1713.97
3
3 1736.5
51973.1 51917.0 1924.07 1926.15
1141 .o 1762.1
;;;;t’; 1823: 13 1825.00
1768.14
60607.8 60551.6 1649.95 1651.48 1066.9
4
61674.7 61618.4 1621.41 1622.89
Comparison intervals
with AG(v+%) vibrational of CN from other sources:
1041.9 5
6
”
A2il
0
1787.71
1214.01
1147.26
1
1762.14
1188.52
1120.99
1018.0
2
1736.56
63734.4 63678.6 1569.01 1570.39
3
1067.28
4
1041.32
5
1016.8
62716.6 62660.4 1594.47 1595.90
i
F2Ar
J2Ai
1093.94
992.7 7
64727.2 64671.2 1544.95 1546.28
Ref.
(18)
(5)
(2), this
ad work
968.5 8
65695.6 65639.8 1522.17 1523.47
Q,,(J)-PI,(J+ with the sum of the known
A
I)-&I(J)+QII(J+
1)
(1)
splittings for the lower state terms J and J + 1, Av,dJ) + Av,AJ+
1).
The labels e and f have their standard meanings (I 7), and the sign of the A splitting is defined by Av~~J) = F’(J) - Fe(J). (3) Expressions (1) and (2) are easily verified to be equal for a Z- upper state, but to have opposite signs if the upper state is F. The experimental evidence derived from the rf
THE L22- +
I 61700 B
I
I
I
1
A*l& TRANSITION
I
I
I
61600 1622.562
413
OF CN
I
I
I 61500
t
I
I 1626.694
C" -I
FIG. 3. The 4-O band of L*Z- + A*Q in emission from the jet discharge. The cold spectrum with T,,,t PZ 40 K is overlapped by a weaker emission of considerably higher temperature.
excited spectrum of the O-2 transition is shown in Fig. 4. Within experimental uncertainties the combination defects (l), represented by open circles, are consistent with the curve corresponding to expression (2), the latter evaluated from the L~*II~,~term values of Cerny et al. (28). The sign of the combination defect clearly points to a Zupper state. Addition of the observed line frequencies to the A*& term values (18) results in upper state energies that reduce to the vibronic energies and rotational constants summarized in Table III. Where the range of observed rotational levels was too limited for a reliable determination of higher-order constants in the least-squares fit, the centrifugal correction D was kept fixed at the calculated value 4Bj/w%. The spectra of the jet discharge have a rotational temperature of approximately 40 K as estimated from an underexposed plate of the 4-O band. In longer exposures like that of Fig. 3 the cold spectrum is usually overlapped by a weaker but rotationally much hotter spectrum which quite possibly originates in the shock regions surrounding the expanding jet. This presents no problems for the analysis; on the contrary, in a few instances it enabled us to extend the measurements to sufficiently high J numbers for the error limits on D to be substantially narrowed. IV. DISCUSSION
The presently available information on the doublet states of configuration . . . is reviewed in Table IV where the equilibrium constants from experimental data are compared with the corresponding ab initio calculated parameters of Schaefer and Heil (7). Although the agreement is only moderately good, the deviations of the calculated from the experimental results are seen to behave in fairly systematic fashion, and it is evident that the new t*I;- state must be associated with the second root of this symmetry obtained by Schaefer and Heil(7), the first root corresponding to a still undetected ‘Z- electronic state at around 60 500 cm-‘. The close correspondence &r3mr
10 11
:
J”-%
%1
51973.12 973.12 972.44 971.19 969.39 967.00 964.03 960.45 956.43
53709.61 709.61 708.80 707.43 705.42 702.83 699.60 695.79 691.26 686.14 680.40 673.92
5544;;:;;
55471.68 471.68 470.76
o-2
437.89 429.81 421.00
5 5463.75 458.41 451.99
o-1
pll
51967.87 965.16 961.83 957.97 953.54 948.48 942.86 936.66 929.86 922.42 914.39 905.93 896.66 aab.73 876.31
51965.16 959.86 953.94 947.37 940.23 932.58 924.30 915.27 905.93 896.04
“-1
_-
53701.66 53704.43 696.35 701.66 698.19 690.32 694. ia 683.56 676.29 689.56 668.41 684.31 659.92 678.47 650.84 671.97 664.82 657.11 648.70 639.67
460.08 455.94 451.16 445.60 439.57 432.90 425.41 417.25 408 52
Q11+p21
%1*Q21
642.36 637.75 632.39 626.28 619.49 611.70 603.30 59L.16 584.28 573.67
66::::: 658.85
928.49 928.95 928.95 927.85 926.25 923.84
646.18 649.19
:66::88:
sIi927.1a
651.42 652.94
648.34 644.99 640.92 635.98 630.33 623.87 616.61 608.63 599.84 590.23 579.97 568.96
:i3650.84
55410.27 406.90 402.65 397.63 391.74 304.99 377.56 369.19
55415.52 415.52 414.84 413.20 410.83 407.59 403.50 398.76 392.88
53653.53 653.53
p22+Q12
Q22+R12
Z::
13658.85
1 661.58
R22
PI2
1 :
0
1:
ii
6’
: 3 4
0
9” 10 11
: 7
;
:
0
:: 12 13 14
2 6’ ; 9
:
0
J”-%
58408.27 410.51 412.10 412.61 412.61 412.10 410.51 408.27
R21
60607.82 607.24 605.68
56612.31 611.35
58400.04 398.93 397.03 394.39 391.09 387.01 382.21 376.76 370.53
*11+921
60602.65 599.64 595.65
796.08
:;9’:;: 572.49
56607.40 604.57 600.92 596.59 591.61
:45::;: 332.34
:%: 378.77 372.89 366.27 358.88
58395.22 392.23
Q11+P21
60600.25 594.61 588.03
3-o
54842.72 837.41 831.25 824.54 817.02 808.94 800.19 790.88
l-2
56599.31 593.14 585.97 578.51
l-l
327.32 315.85
58392.60 387.01 380.67 373.48 365.82 357.26 348.06
1-O
pll
344.65
350.96
58349.35 351.82 353.67 354.21 354.21
R22
Wavenumbers (cm-‘) and Rotational Assignments for Lines in the L*Z- + A211iBands of ‘*C14N
TABLE II
60551.59 551.59 550.28 548.05
54794.53 794.53 793 .a4 792.09 789.62 786.43 782.32 777.51 771.76 765.22 757.84
56556.46 556.46 555.64 553.86 551.29 547.82 543.47 538.29 532.19 525.28 517.57 508.94
ii
58344. 344.11 343.09 341.28 330.54 334.85 330.29 324.84 x8.51 311.23 303.15 294.29 284.58 273.97 262.38
Q22*R12
60549.15 546.44 542.72 537.93
54789.32 706.01 781.72 776.70 770.83 764.09 756.73 748.36
56553.86 551.29 547.82 543.47 538.29 532.19 525.28 517.57 508.94
:‘8: .:t 274156 262.98 250.72
58338.86 335.28 330.81 325.44 319.18 311 .a8
p22+Q12
58315.00 306.08 296.35
P12
i%:::
z:::: 684.76 683.90
61682.33
62716.64 715.76 113.94 711.44 707.83 703.46 698.14 691.97 685.07
61674.65 673.88 672.23 669.73 666.44 662.28 657.35 651 .57 644.96 637.50
565.12
60603.46 600.25 596.69 591.97 586.59 580 .oa
5-o
:
:;:.:: 557 64 542.64 526.46 509.78 491.96 473.31 453.89 433.49 412.15
61667.14 661.43 654.75 641.36 639.07 629.90 620.02 609.40 597.58
4-o
:45::;; 532.21 519.80 506.99
3-o (con1 60580.61 572.62 563.68
62711.68 62709.32 708.35 703.46 704.12 699.04 %: :! 693 09 680.63 686.24 671.54 678.55 661.34 669.92 650.15 660.39 638.38
507.68 575.16 561.41 547.10 531.87 515.88 498.71 480.78 461.93 442.21 421.67 400.20
61669.73 666.44 662.28 657.35 651.57 644.96 637.50 629.19 620.02 609.97
578.80 571.74 563.68 554.97 544.97 534.91 523.67
6”;;;:;;
Ix: 667.59 665.43
62665.43 667.59
:;XZ 583.29
612.66 606. a9
627.02 621.02 626.27 624.37 621.45
61623.57
60552.42 548.05 542.72 536.48
‘d)
62660.39 660.39 658.18 656.34 652.78 648.17 642.55 635.86 628.09 619.47
616.97 614.60 611.21 606. a9 601.53 595.13 587.68 579.42 570.01 559.75 540.42 536.05 522.05 508.67 493.51 477.38 460.20 442.21 423.20 403.23
6’;;;:;;
60544.97 540.73 535.81 529.60 522.55 514.61 505.76
62658.16 655.39 651.48 646.52 640.53 633.49 625.44 616.26 606.02 594.99
K: 584.11 575.16 565.38 554.54 542.64 529.93 515.88 501.30 485.61 468.86 451 .25 432.67 413.04
61616.15 613.24 609.48 604.64
489.39
60532.21 525.63 517.94 509.42
TABLE
i
9 10 11
1: 11 12 13
i
2
3 4
:
0
H-Continued
:Ki: 691.39
:2’:::: 716.90 711.93
64727.la 725.99
:::: :t 720.11 714.56 708.20
63734.35 133.36 731.32
682.71 677.09 670.61 663.12 654.59 645.08 634.49 622.83
65;;;:;:
714.29 708.99 702.71 695.29 687.08 617.89 667.70 656.72 644.44
64;;;::;
716150 710.31 703.30 695.32 686.44 676.51
63129.54 :x
62650.15 638.89
675.61 667.76 658.90
“‘;“,W:;;
a-o
707.17 699.37 690. a2 681.07
ti4;;94:;98
7-o
706.87 698.09 688.81 678.59
63727.03 721.25 714.56
6-O
5-O (cmt’d) 62625.44
65637.43 634.49 630.36 625.16 618.61 610.97 602.15 592 29 581.20
%::I: 576.54
64669.06 666.20 662.18 657.23 651.06 643.40 635.32 625.18 615.15
670.03 665.06 659.26 652.35 644.26 635.11
;;.:;;
63678.59
65637.43 634.49 630.36 625.16 618.61 610.97 602.15 592.29 581.20 568.86 555.56 541 .oo
64669.06 666.20 662.18 656.12 650.56 643.08 634.47 624.88 614.03 602.28 589.09 574.98
657.85 650.76 642.39 633.09 622.58
63;;::;;
416
HUBER, KLUG, AND ALBERT1 cm“ I
- 0.1
0 0 I,
I
IO
I
I
I,
I
20
I
I
J-h
FIG. 4. Combination defects (open circles) according to expression (1) for the L*X- + A*& O-2 transition of CN. The curve represents the lower state A splittings, summed according to expression (2) and evaluated from Ref. (18).
between vibrational and rotational constants for the 2A and 28- states underlines their derivation from a common electron configuration. The E22+ state, on the other hand, deviates considerably from the other four states in the table. This state is strongly mixed with B2Z+ (7); the effects of the perturbation are significantly increased o, and Be values and a strongly anharmonic potential curve. Based on the predicted proximity of the two 2Z- states one might expect the L -+ A system to be accompanied by a second band system of similar appearance, shifted to longer wavelengths but reaching with a long upper state progression well into the region of the L + A bands. This has not been observed; all strong features shortward of 2000 A have now been accounted for, and there is no evidence for emission from high vibrational levels of a second 22- state. This does not preclude, however, the possibility that this state emits at longer wavelengths, in the dense and still incompletely analyzed region from 2000 to 2500 A. A wider than anticipated spectral separation of the two systems could, e.g., come about if the electronic transition moments were strongly r dependent. In this context it is instructive to note that of the two 2A + 211systems the J + A bands [(2), and this work] are largely confined to wavelengths shorter than 2000 A and tend to arise from transitions between high vibrational levels in the initial and low vibrational levels in the final state. By contrast, the F + A bands (1, 4, 5) are prominent at longer wavelengths and combine low vibrational levels of the upper with high vibrational levels in the lower state; they are virtually absent from the shortwavelength region. The Deslandres tables of both systems have lopsided intensity distributions which emphasize only one or the other branch of the Condon parabola, the full parabola becoming apparent, however, when the two Deslandres tables are superposed on each other. Some of the gaps in the observations may, of course, disappear as the analysis progresses toward longer wavelengths. Nevertheless, the available evidence seems to point to marked variations in the electronic transition moments of J2A -+ A211 and F2A -WA211.The J + A system favors transitions at short internuclear
THE L*Z- +
A211i TRANSITION
417
OF CN
TABLE III Molecular Constants for the L*Z- State of ‘*(?‘N from Least-Squares Fits to the Upper State Term Values of the L + A ~‘-1)”Transitions Range of fitted rot. levels (N’)
1_“I1
v
106xD;
T’ ”
Order of spectrum
Discharge
ofit
o-1 o-2 o-2 o-3
l...lO 3...28 0 16 1:::15
66345.96(3) 66345.92(7) 66346.01(2) 66345.97(5)
1.3239(5) i.3238(5) 1.3236(5) 1.3245(11)
7.0 6.8(6) 5.i(ia) 9.1(47)
0.019 0.054 0.016 0.029
Jet R.F. Jet Jet
1-o 1-o l-l l-2
5...29 0...15 O...ll l...ll
67486.93(11) 67486.95(6) 67487.01(6) 67487.10(5)
1.3051(7) 1.3053(5) 1.3037(10) i .3045(a)
7.6(8) 7.0 7.0 7.0
0.066 0.047 0.046 0.035
R.F. Jet Jet Jet
3-o
0...12
69694.59(6)
1.2662(8)
7.0
0.048
Jet
6
4-o 4-o
O...lO 0...23
70761.53(4) 70761.51(4)
1.2457(a) 1.2464(5)
7.0 6.7(9)
0.029 0.037
Jet Jet
7 6
5-o
O...ll
71803.59(6)
1.2279(9)
7.0
0.042
Jet
7
6-O
O...lO
72821.41(6)
i.2085(10)
7.0
0.041
Jet
7
3 3 66 :
7-o
1...14
73814.36(a)
1.1878(a)
7.0
0.061
Jet
7
a-o
O...ll
74782.90(3)
i.l689(5)
7.0
0.022
Jet
7
Note:
-
T(, relative
-
Uncertainties place
and
fixed
at
to
X2E+(v=0, (in
correspond the
N=O).
parentheses) to
calculated
30.
are
in
units
DG values
value
of
without
the error
last
quoted
limits
decimal
were
kept
De=4Be3/we2.
TABLE IV Doublet Electronic States of Configuration . . . (rZr30r.Experimental and Ab Initio Calculated Equilibrium Constants for ‘*C14N State
w e
Te
wx e e
a
*e
Ref.
r,(d)
e
L2E-
talc. obs.
70200 66797.25
1183.2 1165.80
14.04 12.41aa
1.142 1.33367
0.0157 0.01937
1.468 1.3986
(7) This
work
J2A,b 1
talc. obs.
68010 62963.15
1211.7 1174.a9
17.58 13.72’
1.159 1.34706
0.0159 0.02063
1.457 1.3916
(7) This
workd
2z-
talc. obs.
64430
1225.5
14.92
1.165
0.0155
1.453
(7)
F2A e r
talc. ohs.
63550 60095.64
1253.7 1239.50
15.35 12.75
1.152 1.3834
0.0138 0.0187
1.461 1.3732
(7) (l)(5)
E22+
talc. obs.
63360 59151.1a
1717.1 16a1.43
30.57f 3.60
1.322 i .487i
0.0151 0.00649f
1.364 1.3245
(7) (2)(3)
ab’Y
= +o.oo76. dee preliminar12re;;lts. firmed
eA v
by
= +29.18
C/ -
bAv = -28.27 Vibrational C isotope o.as(v+*).
+ 0.594(V+k). numbering
of
‘WeYe = +0.048. Ref.(2) increased
shifts. fweye
= -1.02;
Ye = -0.00077
(v<4).
by +2 and
con-
418
HUBER, KLUG, AND ALBERT1
distances over those taking place at increased r values; in the F + A system the trend is just reversed. The detailed analysis of the emission spectrum longward of 2000 A will show if a similar situation applies to the *Z- states. Unfortunately, it appears that no ab initio calculations of the relevant transition moments have been carried out so far. The observed *2- vibronic levels are remarkably free of perturbations. However, no emission from v = 2 of L*Z-has been seen. In particular, the 2-O band is absent, and this in spite of the fact that both 1-O and 3-O appear with measurable intensity on our plates. The L*Z-,2)= 2 level can be calculated to lie at 68 603 cm-r above X*2+, v = 0 and to coincide almost exactly with the extrapolated position of E*2+, v = 6 at -68 617 cm-‘. Lutz (3) searched for the latter in the absorption spectrum, but, although he observed the E + X d-0 progression up to the relatively strong 50 band, he was unable to make a definitive identification of the 6-O band. The only evidence for absorption near the expected position of 6-O came from a low-dispersion plate where Lutz noticed a diffuse band with a rotational structure not unlike that of the E + X bands. This feature failed, however, to show up under high dispersion. Because of the very close approach of E*2+,o = 6 to L*Z-,o = 2 it is tempting to argue that the observed anomalies in the emission and absorption spectra are the result of an interaction between these two levels. The necessary matrix element is provided by the microscopic form of the spin-orbit interaction operator, ECaili- Si. A perturbation by *X- could readily account for a small shift and weakening of the E*Z+ + X22+ 6-O band. Conversely, the same interaction might be sufficient to wipe out all emission from 2) = 2 of L*Z-if the higher vibrational levels of E*Z+,particularly 21= 6, were subject to weak predissociation. For 2)< 5 they appear with perfectly sharp rotational structure in the absorption spectrum, but it may be significant that no emission has ever been seen from E2Zf levels higher than 2) = 1, the v = 2 level at 62 285 cm-’ being just within the error limits of the best currently available value for the dissociation energy of CN, Dt = 62 500 +- 800 cm-‘. The latter follows from thermochemical data in Refs. (19) and (20). It is obvious that the identification of the LLZ- + ALIIi system in the emission spectrum of CN would have been extremely difficult and time consuming if it had had to rely exclusively on spectra generated by the rf or some other standard type discharge. The comprehensive analysis presented here is proof of the considerable potential of the jet emission technique in the vacuum ultraviolet region as well as in the visible or infrared. For use in the Schumann region the small dimensions of the source may, on occasion, create alignment problems, and the relative weakness of the emission from this source runs counter to the constant need for more intensity in the vacuum ultraviolet, but these disadvantages are more than outweighed by the remarkable spectral purity of the source and by rotational temperatures which-though not spectacularly low by today’s standards-are just about ideal for the performance of a rotational analysis. ACKNOWLEDGMENTS We thank Dr. G. Herzbexg and Dr. J. K. G. Watson for reading this manuscript and for their helpful comments. RECEIVED:
February 2, 1987
THE L*Z- +
&Ii
TRANSITION
OF CN
419
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.
20.
A. E. D~LJGLAS AND P. M. ROUTLY,Astrophys. J. Suppl. Ser. 1,295-317 (1955). P. K. CARROLL,Can&. J. Phys. 34,83-95 (1956). B. L. LUTZ, C’unad. J. Phys. 48, 1192-1199 (1970). B. L. JHAAND D. R. RAo, Proc. Indian Acad. Sci. A 63, 3 16-320 (1966). B. L. LUTZ, Astrophys. J. 164,213-216 (1971). K. P. HUBER ANDG. HERZBERG, “MolecularSpectra and MolecularStructure,IV. Constantsof Diatomic Molecules,” Van Nostrand-Reinhold, New York, NY, 1979. H. F. SCHAEFER AND T. G. HEIL, J. Chem. Phys. 54,2573-2580 (1971). P. J. BRUNA,H. D~HMANN,AND S. D. PEYERIMHOFF, Canad. J. Phys. 62, 1508-1523 (1984). H. ITO,Y. OZAKI,T. NAGATA,T. KONDOW,ANDK. KUCHITSU,Cunod. J. Phys. 62,1586-l 598 (I 984). H. ITO, Y. OZAKI, T. NAGATA,T. KONDOW,K. KUCHITSU,K. TAKATSUKA,H. NAKAMURA,AND Y. OSAMURA,Chem. Phys. 98,8 l-87 (1985). A. T. DROEGEAND P. C. ENGELKING, Chem. Phys. Lett. 96,316-318 (1983); P. C. ENGELKING, Rev. Sci. Instrum. 57, 2274-2277 (1986). K. P. HUBERANDT. J. SEARS,Chem. Phys. Lett. 113,129-134 (1985). K. P. HUBER,M. VERVLOET,CH. JUNGEN,AND A. L. ROCHE,Mol. Phys. (1987), in press. K. P. HUBERAND R. H. LIPSON,J. Mol. Spectrosc. 119,433-445 (1986). G. DAS, T. JANIS,AND A. C. WAHL, J. Chem. Phys. 61, 1274-1279 (1974). H. LAVENDY,G. GANDARA,AND J. M. ROBBE,J. Mol. Spectrosc. 106, 395-410 (1984). J. M. BROWN,J. T. HOUGEN,K. P. HUBER,J. W. C. JOHNS,1. KOPP,H. LEFEBVRE-BRION, A. J. MERER,D. A. RAMSAY,J. ROSTAS,AND R. N. ZARE,J. Mol. Spectrosc. 55,500-503 (1975). D. CERNY,R. BACIS,G. GUELACHVILI, AND F. Roux, J. Mol. Spectrosc. 73, 154-167 (1978). D. D. WAGMAN, W. H. EVANS,V. B. PARKER,R. H. SCHUMM,I. HALOW,S. BAILEY,K. L. CHURNEY, AND R. L. NU~ALL, The NBS Tables of Chemical Thermodynamical Properties, J. Phys. Chem. Ref Data 11, Suppl. 2 (1982). M. W. CHASE,J. L. CURNUTT,J. R. D~WNEY, R. A. MCDONALD,A. N. SYVERUD,AND E. A. VALENZUELA,JANAF ThermochemicalTables, 1982 Supplement,J. Phys. Chem. Ref: Data 11,695940 (1982).