Spectroscopic assignment

Using the vibrational assignment in Section 5.1.1, the observed emission wavenumbers can be used to extract the vibrational constants𝜔

𝑒,𝜔

𝑒𝑥

𝑒 and𝜔

𝑒𝑦

𝑒of the two species.

The term values for an anharmonic oscillator (in cm⁻¹) are given by 𝐺(𝑣) =𝜔 𝜔𝑒is the harmonic frequency. 𝜔

𝑒𝑥

𝑒 and𝜔

𝑒𝑦

𝑒are anharmonicity constants. Based on Eq. 5.1, the corresponding emission wavenumbers for overtone transitions (𝑣→𝑣−2) are

Chapter 5 Laser-induced orientational isomerization in the CO/NaCl(100) monolayer

Figure 5.2: The emission spectrum of a¹³C¹⁸O/NaCl(100) monolayer covered by 100 additional

12C¹⁶O overlayers (buried monolayer), obtained after excitation of the absorption line near 2053.5 cm⁻¹with 145µJ/pulse at a 5.0µA bias current, is shown in red. This is compared to the bare monolayer emission spectrum in Fig. 5.1, shown in black. For both spectra, two vibrational progressions with similar emission frequencies are observed, which are indicated by combs for selected emission lines. The absolute vibrational assignment can be found in Fig. 5.1. From Ref. [126]. Reprinted with permission from AAAS.

By fitting the observed emission wavenumbers to the expression in Eq. 5.2, the spec-troscopic constants𝜔

𝑒,𝜔

𝑒𝑥

𝑒and𝜔

𝑒𝑦

𝑒for the C-down and O-down species in the bare and buried monolayer can be derived. Their values are shown in Table 5.1. In addition, the spectroscopic constants for¹³C¹⁸O and¹²C¹⁶O multilayers, obtained from the corre-sponding multilayer emission spectra, and of the¹²C¹⁶O C-down species, obtained from

12C¹⁶O monolayer emission spectra, are presented (the spectra are not explicitly shown).

The same spectroscopic constants of different isotopologues of gas phase CO are also shown for comparison. For the most relevant species, the extrapolated fundamental frequencies of the𝑣 =0 →1 transitions, ˜𝜈

0→1 = 𝐺(1) −𝐺(0), are calculated based on the listed spectroscopic constants and compared to the corresponding gas phase isotopologue.

Comparison of the harmonic frequencies of the C-down and and O-down species in the bare monolayer with the harmonic frequency of¹³C¹⁸O in the gas phase (𝜔

𝑒 = 2067.80 cm⁻¹) shows frequency shifts in opposite directions: The C-down species

94

5.1 Results

Table 5.1: Vibrational constants𝜔_𝑒,𝜔_𝑒𝑥_𝑒and𝜔_𝑒𝑦_𝑒of various CO isotopologues in the gas phase, in the multilayer, in the buried monolayer and in the bare monolayer. C-down and O-down species are denoted as Na⁺– CO and Na⁺– OC, respectively. ¹³C¹⁸O species are highlighted in grey. In addition, the calculated fundamental frequencies, ˜𝜈_0→1, and the vibrational frequency shifts,Δ˜𝜈_0→1, relative to the corresponding gas phase isotopologue are also given for the bare monolayer and the buried monolayer. From Ref. [126]. Adapted with permission from AAAS.

sample isotopologue/

12C¹⁶O 2166.6±1.0 13.39±0.07 14.8±1.3 2139.8 –

13C¹⁸O 2065.6±1.1 12.19±0.08 13.2±1.5 2041.3 –

buried

∗The gas-phase values for¹²C¹⁶O are taken from Ref. [136] and are actually available with much higher accuracy than shown here. The values for other isotopologues were calculated based on the scaling relations of the Dunham coefficients with the reduced mass𝜇(𝜔_𝑒∝𝜇^−1/2, 𝜔_𝑒𝑥_𝑒∝𝜇⁻¹and𝜔_𝑒𝑦_𝑒∝𝜇^−3/2). The rounded values for¹³C¹⁶O and¹²C¹⁸O calculated in this way agree extremely well with the experimental data by Toddet al., which indicates that the values are accurate for all isotopes within the given precision.

†The error bars represent 95 % confidence intervals (2𝜎). They were derived by calculating the weighted average of the spectroscopic constants and the corresponding errors from several fits to different measurements except for the buried monolayer where the constants were derived from a single spectrum shown in Fig. 5.2.

‡Note that this value is in excellent agreement with the experimental value of 2150.5 cm⁻¹found when¹²C¹⁶O is diluted in¹³C¹⁶O [107], conditions where the influence of the excitonic splitting in absorption spectroscopy is essentially removed. In emission spectroscopy, the splitting also does not appear. We have observed emission lines for the O-down isomer of¹²C¹⁶O, but the data is not sufficient to derive its spectroscopic constants accurately.

Chapter 5 Laser-induced orientational isomerization in the CO/NaCl(100) monolayer (𝜔

𝑒=(2075.7±0.7)cm⁻¹) is blue-shifted with respect to the gas phase value while the O-down species (𝜔

𝑒=(2058.8±0.9)cm⁻¹) is red-shifted. The blue- and red-shifts are, of course, also reflected in the fundamental frequency shifts,Δ𝜈˜

0→1, relative to the gas phase frequency. Furthermore, the extrapolated fundamental frequency of the C-down species in the bare monolayer (2051.3 cm⁻¹) is consistent with the Davydov doublet at 2049 and 2055 cm⁻¹observed in absorption. The same trends are found for the buried monolayer. Because the bare monolayer system is less complex and its spectroscopic constants are obtained with higher accuracy, the following analysis will mostly focus on the bare monolayer.

Since isotopically enriched¹³C¹⁸O has been used for the experiments with less than 1 %¹³C¹⁶O and¹³C¹⁷O impurities (see also Section 3.1.5), it is reasonable to assume that the vibrational progressions observed in the monolayer emission spectrum can be assigned to two different ¹³C¹⁸O species. This hypothesis is supported by the clear mass dependence seen in the gas phase spectroscopic constants of the various CO isotopologues listed in Table 5.1. To be more specific, the following scaling relations apply, [136] where𝜇is the reduced mass of the molecule:

𝜔𝑒∝ 𝜇^−1/2, 𝜔

𝑒𝑥

𝑒∝ 𝜇⁻¹and𝜔

𝑒𝑦

𝑒 ∝𝜇^−3/2. (5.3)

Thus, the spectroscopic constants show characteristic values depending on the isotopo-logue and can be used to assign the C-down and O-down species in the bare monolayer to¹³C¹⁸O (see Table 5.1). The harmonic frequencies of both species are clearly located near the¹³C¹⁸O gas phase value. In addition,𝜔

𝑒𝑥

𝑒 ≈12.2 cm⁻¹is only 0.1 cm⁻¹larger than the ¹³C¹⁸O gas phase value and identical for both species within experimental errors; the difference to the next isotopologue (¹³C¹⁷O) is slightly larger (∼0.15 cm⁻¹).

A similar value of𝜔

𝑒𝑥

𝑒 is also observed for the ¹³C¹⁸O multilayer, providing further evidence that both species can be assigned to different ¹³C¹⁸O species. To definitely exclude that the O-down progression can simply be assigned to a C-down progression of a different isotopologue, the spectroscopic constants of Na⁺–¹³C¹⁸O are used to predict the spectroscopic constants of Na⁺–¹³C¹⁶O and Na⁺–¹³C¹⁷O using the isotopic relations in Eq. 5.3. The predicted constants can then be used to predict the corresponding emission frequencies (Eq. 5.2). Figure 5.3 clearly shows that the predicted emission frequencies of Na⁺–¹³C¹⁶O and Na⁺–¹³C¹⁷O do not agree with the observed emission peaks of the O-down species.

The spectroscopic constants in Table 5.1, in particular the resulting red-shift for the O-down isomer, strongly rely on the vibrational assignment in Fig. 5.1. The infrared absorption spectra of the buried monolayer in Fig. 5.4 give strong evidence that the given

96

5.1 Results

3200 3400 3600 3800 4000

Emission frequency (cm ) 0

1 2 3 4 5 6

Intensity (arb. u.)

13C¹⁸O (fit)

13C¹⁷O (predicted)

13C¹⁶O (predicted)

Figure 5.3: Black lines indicate the emission frequencies calculated with the fitted spectroscopic constants of the C-down species in the bare monolayer (Na⁺–¹³C¹⁸O). Blue and red lines indicate predictions for the emission peak positions of Na⁺–¹³C¹⁷O and Na⁺–¹³C¹⁶O based on the isotopic relations in Eq. 5.3 and the spectroscopic constants of Na⁺–¹³C¹⁸O.

assignment is indeed correct. Figure 5.4a shows the polarized Fourier-transform infrared (FTIR) absorption spectra of the freshly prepared buried monolayer sample, measured at 7 K. The spectra show an intense peak at 2053.5 cm⁻¹and a tiny peak at 2048.2 cm⁻¹, both of which are observed with p- and s-polarization. The intense peak is consistent with the absorption peak at 2153.6 cm⁻¹reported for a buried¹²C¹⁶O monolayer, [44]

considering that the fundamental frequencies of¹³C¹⁸O and¹²C¹⁶O differ by almost exactly 100 cm⁻¹. The low intensity peak, on the other hand, has not been reported previously. Observation of a doublet can be explained by exciton splitting if the unit cell consists of two inequivalent CO molecules, similar to the doublet observed for the bare monolayer (see also Section 2.1). [36,38] Based on simulations of the absorption spectra, we estimated the structure of the buried monolayer in the Supplementary Materials of Ref. [126].¹ In addition, this simulation provided an estimate of the fundamental frequency if the dynamic exciton splitting was not present. The estimate of this unsplit frequency is indicated in Fig. 5.4 and agrees well with the extrapolated fundamental frequency value obtained from the C-down emission lines and indicated by the blue bar.

1A slightly better estimate for the tilt angle of the C-down and O-down species is shown in Appendix C.

Chapter 5 Laser-induced orientational isomerization in the CO/NaCl(100) monolayer However, no absorption peak is observed for the red-shifted O-down species before laser excitation.

Following laser excitation of the¹²C¹⁶O overlayer,²a second absorption peak appears at∼2037 cm⁻¹, accompanied by a redshift and broadening of the C-down peak (Fig. 5.4b).

This second peak is in good agreement with the extrapolated fundamental frequency of the O-down species indicated by the red bar, thus confirming the vibrational assignment of the two species made above. At 7 K, no change in the O-down absorption peak is observed over the course of 12 h but the laser-induced change can be fully reversed by annealing at 22 K for 20 min (Fig. 5.4c). Only the long lifetime of the O-down species makes the observation with conventional absorption spectroscopy possible in the buried monolayer case. For the bare monolayer, the red-shifted species could not be observed in absorption, suggesting that its lifetime at 7 K is shorter than the typical time required to measure an FTIR spectrum (approx. 5 min), which is significantly shorter than the lifetime observed for the buried monolayer.