\documentclass[twocolumn]{article}
\begin{document}
\newcommand\omicron{o}
\begin{table}[htb]\caption{
}\vspace{12pt}\centerline{\begin{tabular}{ll}
\hline
a & $\alpha$   \\
b & $\beta$    \\
c & $\gamma$   \\
d & $\delta$   \\
e & $\epsilon$ \\
f & $\zeta$    \\
g & $\eta$     \\
h & $\theta$   \\
i & $\iota$    \\
j & $\kappa$   \\
k & $\lambda$  \\
l & $\mu$      \\
m & $\nu$      \\
n & $\xi$      \\
o & $\omicron$ \\
p & $\pi$      \\
q & $\rho$     \\
r & $\sigma$   \\
s & $\tau$     \\
t & $\upsilon$ \\
u & $\phi$     \\
v & $\chi$     \\
w & $\psi$     \\
x & $\omega$   \\
y & $\infty$   \\
z & $\ell$     \\
\label{Ttimescale}\end{tabular}}\end{table}
\end{document}
