Laser-induced fluorescence measurements

This section describes the measurement conditions and procedures to obtain time-resolved measurements of single vibrational states as well as time-integrated excitation and emission spectra using LIF. Photon detection events from the SNSPD are counted by the MCS as a function of time (see also Section 3.1.2). The MCS has a maximum time-resolution of 100 ps and, using a picosecond laser, we have achieved sub-nanosecond time resolution with an SNSPD similar to the ones used in this work. [93] For the experiments presented here, the time resolution is limited by the temporal width of the the pulsed nanosecond laser (4.7 ns FWHM). [20] Thus, data is typically collected with a comparable bin time of 12.8 ns, defining the theoretical time resolution. However, the effective resolution of the experiments depends on the SNR; while excitation of

62

3.2 Experimental procedures samples with 50-100 layers can realistically provide sufficient SNR at 12.8 ns resolution, excitation of a monolayer typically requires rebinning of the data for a comparable SNR, effectively lowering the resolution. The fluorescence signals are recorded up to∼50 ms after laser excitation because rates for vibrational relaxation in CO/NaCl(100) are small and can be on the order of 10 s⁻¹.

Typical time profiles, measured for fluorescence from a single emission line, are shown in Fig. 3.12. Figure 3.12a illustrates an example of a time profile measurement observing a high intensity emission line after excitation of a¹²C¹⁶O monolayer covered by 100¹³C¹⁸O overlayers, which gives the best SNR for excitation of a single layer (see also Chapter 6). With 12.8 ns bins, barely any trend can be observed; rebinning to 640 ns allows a rising time of about∼10µs to be resolved. 𝑡=0 corresponds to the arrival time of the excitation laser pulse which can be readily determined by observing stray light from the excitation laser, which is blocked here using optical filters (see below for further details). Fig. 3.12b, on the other hand, shows the temporal decay for an emission line with about 10 times smaller intensity observed when a ¹³C¹⁸O monolayer covered by 100¹²C¹⁶O overlayers is excited (see Chapter 6), which is at the limit of what can still be clearly seen in the corresponding emission spectra. In this case, the initial rising part is not resolved within the 640 ns resolution and the SNR is obviously worse than in the first example.

Time-integration of these profiles allows measurements of LIF emission spectra, i.e., emission frequency is varied at a fixed excitation frequency, or excitation spectra, i.e., the excitation frequency is varied at a fixed emission frequency. Integration time windows are chosen such that they cover different parts of the time-dependent signal. The size of the integration windows ranges from 10µs and 9 ms but the lower integration limit of each window is identical and placed close to the arrival time of the excitation laser pulse (𝑡 =0), either at 0.005 or 0.05 ms (illustrated in Fig. 3.12c). Background is recorded simultaneously by integrating over the 45-50 ms window after laser excitation, in which no fluorescence signal is observed. Because time-integrated spectra do not require the 12.8 ns time resolution used for time-resolved measurements of a single emission line, data is acquired with lower 819.2 ns resolution to reduce the amount of data transferred from the MCS and thus speed up the data acquisition process.

The measured time profiles reflect the kinetic traces of the measured vibrational states as long as the maximum number of counts per bin (i.e., the event count rate) is below a threshold that avoids overlapping detection events. This maximum count rate can be determined from the recovery time of the SNSPD. The dead time of the detector extends over approx. 400 ns, which corresponds to count rate 2.5×10⁶counts s⁻¹. To avoid two detection events within the dead time, the SNSPD is operated with a bias current

Chapter 3 Experimental

- 5 0 0 5 0 1 0 0 1 5 0 2 0 0 2 5 0

0 2 5 5 0 7 5 1 0 0

Counts / (1000 laser pulses)

T im e ( µ s )

e x c i t a t i o n p u l s e ( t = 0 ) ( a )

- 5 0 0 5 0 1 0 0 1 5 0 2 0 0 2 5 0

0 . 0 2 . 5 5 . 0 7 . 5 1 0 . 0

Counts / (1000 laser pulses)

T im e ( µ s )

e x c i t a t i o n p u l s e ( t = 0 ) ( b )

0 2 4 6 8 1 0 4 4 4 6 4 8 5 0

0 2 5 5 0 7 5 1 0 0

Counts / (1000 laser pulses)

T im e ( m s )

b a c k g r o u n d w i n d o w 1 0 m s w i n d o w

2 m s w i n d o w ( c )

Figure 3.12: (a) Temporal profile for a high intensity emission line in the region where laser excitation occurs (𝑡 =0). The line corresponds to the 3677 cm⁻¹ emission line observed for excitation of a¹²C¹⁶O monolayer covered by¹³C¹⁸O overlayers (see Chapter 6). (b) Temporal profile for a low intensity emission line that can still be clearly detected in the emission spectrum.

The line corresponds to the 3695 cm⁻¹emission line observed for excitation of a¹³C¹⁸O monolayer covered by¹²C¹⁶O overlayers (see Chapter 6). (c) The same temporal profile as in a) is shown for long times up to 50 ms. In addition, selected integration time windows for time-integrated excitation and emission spectra are indicated. Measurement windows start shortly after the laser pulse (here: 0.05 ms) to avoid integration over𝑡 =0, which may contain contributions from scattered light. For demonstration purposes, only 2 and 10 ms windows are shown. For background, the 45-50 ms time window is used. All time profiles share the same bin size of 640 ns.

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3.2 Experimental procedures that ensures a maximum count rate of 10⁵counts s⁻¹ that is one order of magnitude smaller. For a typical bin time of 640 ns accumulated over 1000 laser pulses, this would correspond to a maximum number of 64 counts, which is not exceeded for the high intensity emission line in Fig. 3.12a.

In addition, double counting of the peaks should be avoided. This can result from noise during the detector dead time which triggers another detection event on the MCS.

The amplified pulse shape in the current setup is clearly distorted by the amplifier circuit, as apparent in Fig. 3.13. It consists of two sharp peaks followed by a broad oscillatory pattern that reflects the 400 ns dead time of the SNSPD. Setting the detection threshold of the MCS at the value indicated in Fig. 3.13 helps to avoid double counting while the distorted pulse shape does not compromise the temporal resolution. The rising edge of the pulse (Fig. 3.13b) extends over only 2 ns and the observed timing jitter, observed by monitoring several detection events with the oscilloscope, is similarly small. This 2 ns uncertainty is likely limited by the 500 MHz bandwidth of the oscilloscope (Wavejet 354A, LeCroy). Thus, the actual timing jitter is expected to be much smaller than the limit to the temporal resolution imposed by the laser pulse.

0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0

Figure 3.13: (a) Oscilloscope trace of a single, amplified SNSPD voltage pulse measured at a bias current of 5.4µA. The dashed line indicates a typical detection threshold value used for the MCS, which avoids double counting of pulses. (b) Rising edge of the pulse presented in (a).

Optical filters can be applied at the entrance and exit slits of the monochromator to block unwanted frequencies (see also Section 3.1.2). In this work, mainly the overtone emission region of the C-O stretching vibration, ranging from∼4250 to 2600 cm⁻¹, is used as a compromise between large SNSPD detection efficiencies at short wavelengths and comparatively small background from thermal background radiation that originates from the UHV chamber. Thus, an optical bandpass filter (center wavelength: 3100

Chapter 3 Experimental

nm, bandwidth: 1300 nm, Andover) for this spectral region is used. The bandpass filter is particularly important for blocking light with frequencies above∼5500 cm⁻¹, which would otherwise be observed as second-order peaks in the CO overtone region.

Furthermore, an optical longpass filter (PW-S-00501, 3 mm thick Infrasil, Laseroptik) is used to block stray light from the excitation laser pulse that is scattered into the detector, which can potentially saturate the SNSPD and thus effectively lower the time resolution near𝑡=0. The transmission spectra of these optical filters, as measured with the FTIR spectrometer, are presented in Fig. 3.14.

1 0 0 0 2 0 0 0 3 0 0 0 4 0 0 0 5 0 0 0 6 0 0 0 7 0 0 0 8 0 0 0

- 2 0 0 2 0 4 0 6 0 8 0 1 0 0 1 2 0

Transmittance (%)

F r e q u e n c y ( c m⁻¹) e x c i t a t i o n f r e q u e n c i e s C O o v e r t o n e e m i s s i o n r e g i o n

( v = 2 t o v = 3 5 )

Figure 3.14: Infrared transmission spectra of the optical filters that are used to remove artifacts that appear due to second-order diffraction or stray laser light. The positions for¹³C¹⁸O and

12C¹⁶O excitation at∼2050 and∼2150 cm⁻¹, respectively, are indicated by arrows. In addition, the C-O overtone spectral region is indicated.