J . Phys. Chem. 1992, 96, 5337-5343
Spectroscopy and Photophysics of the 'Au C,HD, and C4D2
+
5337
'2,' Transition of Jet-Cooled C4H2,
Ralph E. Bandy, Chitra Lakshminarayan, and Timothy S. Zwier*vt Department of Chemistry, Purdue University, West Lafayette, Indiana 47907- 1393 (Received: February 10, 1992; In Final Form: March 30, 1992)
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Resonant two-photon ionization spectra of the 'A, IZg+ transition of C4H2, C4HD,and C4D2 cooled in a supersonic molecular beam have been recorded and partially analyzed. The cooling provided by the supersonic expansion reveals several vibronic transitions not resolved at room temperature. While some progress is made on this basis in the vibronic assignments, the full analysis of the spectra is hindered by the considerable complexity of the upper state spectroscopy involving both Renner-Teller and Herzberg-Teller coupling. Only vibronically induced transitions are observed in the spectra. The absence of the 2n0 transitions indicates that the vibronic spectroscopy can be treated assuming a linear or quasi-linear upper state geometry. The observation of the 2n0transitions in past matrix studies must be ascribed to matrix effects. The narrowing of the rotational band contours in the molecular beam allows a study of the single vibronic level broadening inherent in the transitions. The full width at half-maxima of the transitions in C4H2 increase from 5 cm-' at 6'0 to 40 cm-I at 220610,indicating strong, energy-dependent coupling of the vibrational levels in the IAu state to a dense bath of background levels. The widths of corresponding transitions in C4H2, C4HD, and C4D2 generally decrease with increasing deuteration, indicating that H-atom motion plays an important role in the coupling. There are also dramatic differences in width from one vibronic level to the next. Some suggestions are made as to the source of these differences.
I. Introduction
This paper has as its focus the spectroscopy and excited-state photophysics of diacetylene, C4H2, in the region from 250 to 217 nm. In undertaking such a study, we are motivated by the important role played by C4Hz in many planetary atmo~pheresl-~ and in combustion proce~ses.~C4Hz is the most complex hydrocarbon which has been clearly identified in planetary atmospheres other than our own, most notably on Saturn's moon Titan. In these atmospheres, C4H2 is proposed as a precursor to more complex hydrocarbons in that it absorbs over much of the ultraviolet region and is known to be photochemically very reactive. Thus, room temperature photochemical studies have been carried out recently on C4H2 by Glicker and OkabeSs They observed a photochemical quantum yield of two for C4Hz throughout the ultraviolet region, despite being unable to detect a C4H photolysis product until excitation wavelengths well below 200 nm. These authors suggest a metastable state of C4H2 as responsible for the observed loss of C4H2,but the products of this photochemistry are still not known. Diacetylene is also important in the comparison it provides with acetylene and higher linear, conjugated C,H2 polyynes. By studying the spectroscopy and photophysics of the polyynes, we are afforded a step-by-step view of the effects of carbon chain and conjugation length on their spectroscopy and photophysics. C2H2has a corresponding lZg+-lAUtransition beginning at 1900 A, which serves as a useful point of comparison with the present study.6 There have been several previous studies of the ultraviolet spectroscopy of C4H2 and C4D2. Hardwick and Ramsay have analyzed several sharp bands in the weak A(IA,)-X('Zg+) transition near 2860 A. The rotational analysis indicates that the ]A, upper state is trans-bent.' Four previous studies have been carried out on the B('A,)-X'Zg+ transition which is our focus, three at room temperature8-l0 and one in a matrix." While some progress has been made in the vibronic assignments over the course of these studies, even the latest assignments have little unassailable supporting evidence. The assignments are hindered by the inherent broadness of the transitions which preclude using rotational structure in the assignments. As we will see, the spectrum is also complex in several other ways. The lack of an origin renders some uncertainty to all vibrational frequencies. Furthermore, the excited-state vibronic levels undergo both Herzberg-Teller and Renner-Teller coupling, so that assignments based on normal-
* To whom correspondence should be addressed. 'Alfred P. Sloan Fellow.
mode analysis produce little headway. This paper presents a single vibronic level resonant two-photon ionization (R2PI) study of C4H2 cooled in a supersonic expansion. The mass selection afforded by time-of-flight analysis of the ions provides spectra of C4H2, C4HD, and C4Dzfree from interference from one another. Furthermore, the cooling of the expansion sharpens the vibronic transitions to resolve even close-lying features. The results thus have two foci. First, we report (section 1II.A) our spectroscopic assignments of the vibronic transitions, building on previous studies. Second, we focus (section 1II.B) on the breadths of the transitions as a measure of the strong coupling of the 'A, single vibronic levels (SVLs) to background states. We will see that there are significant differences in the strength of this coupling with changing total energy, vibronic level, and isotope. These variations provide clues to the types of states responsible for the coupling. 11. Experimental Section
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One-color resonant two-photon ionization (R2PI) is used to record Sz So spectra using a molecular beam timeof-flight mass spectrometer (TOFMS) described previously.I2 Mixtures of diacetylene in helium (1-2%) at total pressures of 2 bar are expanded through a 0.8-mm-diameter nozzle, skimmed, and crossed in the ion source of a linear TOFMS with the doubled output of an excimer-pumped dye laser. Coumarin 480,460, and 440 dyes are used to cover the wavelength range from 250 to 217 nm. The ultraviolet light (-0.1-0.5 mJ/pulse) was focused with a 25-cm-focal length lens into the ion source. In order to avoid excessive fragmentation and power broadening in the spectra, the lens position was typically placed 28-30 cm from the center of the ion source. Estimated peak power densities are in the range (1-5) X lo9 W/cm2. In this range, the breadths of the transitions were independent of laser power. We estimate the rotational temperature of the expansion to be no more than 5 K based on previous experience with other molecules at the same expansion conditions.I2 Spectra of the isotope of interest are recorded by monitoring the appropriate mass in the TOF mass spectrum using a digital oscilloscope (LeCroy 9400) under computer control. Typical ion signals were only about one ion per laser pulse. C4H2 is prepared using the procedure of Armitage et al.I3 and collected in a cold trap at -70 OC. Since the C4H2 is highly unstable as a pure liquid, the sample is immediately transferred to a gas cylinder by slowly warming the sample under vacuum with the cylinder attached to the vacuum line. The gas cylinder is then filled to a total pressure of 7 bar with helium to produce
0022-3654/92/2096-5337%03.00/0 0 1992 American Chemical Society
Bandy et al.
5338 The Journal of Physical Chemistry, Vol. 96, No. 13, 1992
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TABLE I: Excited-State Frequencies and Assignments of the Observed Transitions in the IAU IES+ RZPI Spectrum of C,HH,, C,HD, and C,D, relative frequency (cm-I)" C4H2 C4HD C4D2 assignment 222 199 7',, (A) 288 254 240 6'0 (B) 424 bending mode (C) 428 426 655 540 535 overtones or (D) 703 combinations (E) 746 753 821 854 892 893 971 1060 928 1132 1252 1198 1179 1363 1436 1382 1466 1634 1592 21 18 2262 2299 2302 2393 2343 2487 2555 2523 2591 2782 2636 2742 2800 3193 3325 3319 4318 4370 4347 4470 4405 4536 4675 4608 4630
I
41000
42000
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44000
43000
45000
46000
Frequency (cm ')
Figure 1. Overview R2PI scans of (a) C4H2, (b) C4D2 and (c) C4HD in IZg+transition. The arrow in (a) marks the the region of the IA, position of the ionization threshold in a one-color R2PI experiment. The laser power spectra over the spectral regions of interest are shown on top.
a mixture which is 1-2% C4H2 in helium. C4D2 is prepared by substituting D 2 0 and NaOD for the protonated species in the synthetic procedure. Mixtures of C4H2, C4HD, and C4D2 can be prepared with varying amounts of the isotopes by adjusting the percentage of deuterated reagents used in the procedure. These mixtures were stable over the duration of the gas fill. 111.
Hot Bandsb
Results and Analysis
A. Spectroscopy of the IAu
+
lZg+Transition. Figure l a - c
presents overview R2PI scans of C4HZ,C4D2, and C4HD, respectively. Several features of these spectra should be highlighted at the outset. First, the relative intensities of the transitions, especially in the lower energy region, do not accurately reflect absorption strengths of the vibronic transitions, since these transitions have energies just over halfway to the ionization threshold of diacet~1ene.I~ The wavelength required to reach the ionization threshold in a one-color R2PI experiment is marked in Figure la with an arrow. The result is that the relative intensities of the lowest transitions reflect a convolution of the IAu IZg+Franck-Condon factors with the Franck-Condon factors for the ionization step. Higher energy transitions reduce these threshold effects, producing relative intensities close to those in previous absorption studies.8-'0 Second, even in a long-range scan such as this (covering more than 6000 cm-I), one can readily observe the finite breadths of the transitions which increase substantially with increasing total energy. Third, despite the fact that many of the transitions are broad, they are nevertheless much sharper than those recorded previously at room temperature?-I0 thus providing a better measure of the inherent breadths of the transitions. Table I presents a list of the observed transitions together with tentative assignments for many of the major features. Included in the table are hot band transitions in C4H2and C4Dz taken from Haink and Jungen9 and Lamotte et al.,IOrespectively. Table I1 summarizes the ground-state frequencies and assignments of C4H2, C4HD, and C4D2 taken from Owen et al.I5 which have been used in previous assingments of some of the vibronic transitions. 1 . An Overview of C.,H2 Spectroscopy. From the positions of the transitions listed in Table I, it is clear that a discussion of spectra in terms of harmonic vibrational frequencies and normal-mode isotope effects provides little help in assigning the low-frequency transitions. As Figure 2 shows, in the linear configuration, the S2 excited state is characterized as IA,. The IA,, state will be Renner-Teller a c t i ~ e lalong ~ , ~ ~the four degenerate bending modes (Table 11) whose vibrational angular momenta can couple to the electronic angular momentum of the excited state. In C2H2,only two such modes exist involving symmetric and antisymmetric CCH bends. In C4H2, distortion can occur along both carbon framework (CCC bend) and terminal hydrogen
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-477 -248 -477
-62 1 -36 1 -475
Frequencies relative to the origins which are estimated to be at 40845 cm-I for C4H2, 40886 cm-l for C4HD, and 40921 cm-' for C,D2. See text for choice of origins for C4H2 and C4D2. The origin for C4HD was chosen so that the relative frequencies of peaks B and C are between those of C4H2 and C4D2. bHot band assignments for C4H2 were taken from ref 9 and those for C4D2 from ref 10. TABLE 11: Ground-State Vibrational Frequencies of C4H2,C,HD, and C4D2" freauencv (cm-l) vibration descriptionb C4H2 C4HD C4D2 Y, C-H(D) 3332 3332 2605 2189 v. c=c 2146 2067 872 u; c-c 839 854 3333 Y, C-H(D) 2598 2600 2019 1938 1890 Y, c=c 626 6, C-H(D) 627 500 483 47 1 459 6, c*-c 628 6, C-H(D) 499 497 220 6, e c - c 210 202 "The ground-state frequencies are taken from ref 15. bNotation: us = symmetric stretch; Y, = asymmetric stretch; 8, = symmetric bend; 8, = asymmetric bend.
atoms (CCH bend). Motion along any of these bending coordinates thus gives rise to a splitting of the upper electronic state into two components (Figure 2), and the progressions involving these modes will be broken up into several components by the coupling between the vibrational (I) and electronic (A) angular momenta. A schematic diagram of the positions of these levels for a single bending mode with different u and K = A + 1 is shown ince in F w e 3 for the case of moderate Rennel-Teller coupling. s the four Renner-Teller active bending modes are far lower in frequency than the other five normal modes (Tables I and 11), the bendmg modes will be responsible for all low-frequency activity in the spectra. These same bending modes also support all vibronically induced intensity in the IA, IZg+ transition via Herzberg-Teller Coucarry oscillator pling. Higher states of IZ,+or 'nu strength from the ground state (Figure 2). These states can induce
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The Journal of Physical Chemistry, Vol. 96,No. 13, 1992 5339
Jet-Cooled C4H2, C4HD, and C4D2
7 x5 a,
I
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/
i
L
$
40
a
20
0 43000
43500
44000
Frequency (cm-')
Figure 4. R2PI scan of C4H2 in the 2'06'0 region. The upper trace shows a close-up of the region at 5 times higher sensitivity. The arrow marks the position where the 210 transition is expected.
W 0 Bending Coordinate
Figure 2. Schematic energy level diagram of a select subset of the electronic states of diacetylene of interest here. Renner-Teller splitting for the 'A, and I l l , states are shown assuming small Renner-Teller coupling. - .e
'\-
v=l
---v
