Figure 1

(Color online) (a) Schematic sketch and (b) energy diagram of a STM junction, in which a single magnesium porphine (MgP) [its molecular structure shown in (c)] molecule is adsorbed on a thin insulating alumina film grown on a NiAl(110) surface. The STM is not only used to study this molecule via topographic imaging and tunneling spectroscopy but it is also used to probe its intramolecular radiative transition by locally exciting the molecule with tunneling electrons. Along with the intramolecular radiative transition (1), other pathways [(2) and (3)] of the tunneling electrons are marked in the diagram (details are discussed in the text).Reuse & Permissions

Figure 2

(Color online) (a) $dI/dV$ spectra measured over a single MgP molecule adsorbed on an oxidized NiAl(110) surface. The tunneling gap was set at $V_b=3.0\text{V}$ and $I=0.1\text{nA}$. (b) A sequence of the emission spectra collected from the same molecule in (a) as a function of the bias voltage $V_b$ marked on each spectrum. An emission spectrum (bottom) with the STM tip positioned on the bare oxide $\left(V_b=2.50\text{V}\right)$ is also shown for comparison. All spectra were acquired with $I=0.1\text{nA}$ for 300 s and are vertically offset for clarity. Individual vibronic peaks were observed with higher bias voltages and are marked in the top spectrum. The photon energy of each of the identified peaks is plotted in (c) as a function of the peak number. The slope yields a vibronic energy of 46 meV. (d) The intensity of the first vibronic peak $\left(h\nu =1.55\text{eV}\right)$ varies with the bias voltage. The profile is fitted with a Gaussian function, determining the peak at $2.79\pm 0.01\text{V}$. This plot is also shown within a larger bias range in (a).Reuse & Permissions

Figure 3

(Color online) A high resolution emission spectrum from a different MgP molecule on the oxide surface. $V_b=2.2\text{V}$, $I=0.1\text{nA}$, and exposure $\text{time}=300\text{s}$. It shows a sharp molecular emission line with a few side peaks. The dominant peak is fitted with a Lorentzian function, giving a FWHM of 4.4 meV.Reuse & Permissions

Figure 4

(Color online) High resolution $dI/dV$ and $d^{2}I/d{V}^2$ spectra measured on the same molecule as in Fig. 2. No vibronic states were resolved by tunneling spectroscopy. The tunneling gap was set at $V_b=1.60\text{V}$ and $I=0.47\text{nA}$. The bias modulation was 10 mV (rms) at 400 Hz.Reuse & Permissions

Figure 5

(Color online) Spatial dependence of the emission spectra from the same molecule as in Fig. 2. The locations of the STM tip where each spectrum was collected are marked in the STM image of this molecule (inset). All spectra were acquired with $V_b=3.0\text{V}$ and $I=0.1\text{nA}$ for 300 s, and are vertically offset for clarity. The STM image (27 by $27{\text{Å}}^2$) were taken at $V_b=2.25\text{V}$ and $I=0.1\text{nA}$. The ripples in the image are due to the coupling of the STM to the mechanical vibrations from the ground; the STM table was not floated when this image was acquired.Reuse & Permissions