I made a table spanning the top of a two column page, but there is a problem in the last column where $j$ is very close to the hline above, while other elements in the same row are of the same vertical height. The same problem happens to the "transition" header in the last column. And is there a way to make the first row a little wider(the row with $\lambda$ and transition)? How to fix these issues?
\newcolumntype{M}{>{\centering\arraybackslash}m{\dimexpr.096\linewidth-2\tabcolsep}}
\begin{table*}[t!]
\footnotesize
\centering
\begin{tabular}{|| M | M || M | M || M | M || M | M || M | M ||}
\multicolumn{2}{c}{\bf Lyman} & \multicolumn{2}{c}{\bf Balmer} & \multicolumn{2}{c}{\bf Paschen} & \multicolumn{2}{c}{\bf Brackett} & \multicolumn{2}{c}{\bf Pfund} \\
\hline
$\lambda$(nm) & transition & $\lambda$(nm) & transition & $\lambda$(nm) & transition & $\lambda$(nm) & transition & $\lambda$(nm) & transition \\[1mm]
\hline
93.78 & 6$\rightarrow$1 & 410.2 & 6$\rightarrow$2 & 1094 & 6$\rightarrow$3 & 2630 & 6$\rightarrow$4 & 7400 & 6$\rightarrow$5 \\[1mm]
94.98 & 5$\rightarrow$1 & 434.0 & 5$\rightarrow$2 & 1282 & 5$\rightarrow$3 & 4050 & 5$\rightarrow$4 & & \\[1mm]
97.25 & 4$\rightarrow$1 & 486.1 & 4$\rightarrow$2 & 1875 & 4$\rightarrow$3 & & & & \\[1mm]
102.6 & 3$\rightarrow$1 & 656.3 & 3$\rightarrow$2 & & & & & & \\[1mm]
121.6 & 2$\rightarrow$1 & & & & & & & & \\[1mm]
\hline
$\frac{\lambda_{1,j}}{\lambda_{1,j+1}}$ & $j$ & $\frac{\lambda_{2,j}}{\lambda_{2,j+1}}$ & $j$ & $\frac{\lambda_{3,j}}{\lambda_{3,j+1}}$ & $j$ & $\frac{\lambda_{4,j}}{\lambda_{4,j+1}}$ & $j$ & $\frac{\lambda_{5,j}}{\lambda_{5,j+1}}$ & $j$ \\[4mm]
\hline
1.01 & 5 & 1.05 & 5 & 1.17 & 5 & 1.54 & 5 & & \\[1mm]
1.02 & 4 & 1.12 & 4 & 1.46 & 4 & & & & \\[1mm]
1.05 & 3 & 1.35 & 3 & & & & & & \\[1mm]
1.18 & 2 & & & & & & & & \\[1mm]
\hline
\end{tabular}
\caption{Expected hydrogen wavelengths, transitions and wavelength ratios.}
\end{table*}
Here is the preamble.
\documentclass[aps,prl,reprint,twocolumn,secnumarabic,nobalancelastpage,amsmath,amssymb,nofootinbib]{revtex4}
% nofootinbib is another document class option that allows you to put
% footnotes on the page where they occur rather than at the end of the
% paper. This makes for easier reading!
% secnumarabic is a particularly nice way of identifying sections by
% number to aid electronic review and commentary.
% amsmath and amssymb are necessary for the subequations environment
% among others
\usepackage{graphics} % standard graphics specifications
\usepackage{graphicx} % alternative graphics specifications
\usepackage{longtable} % helps with long table options
\usepackage{url} % for on-line citations
\usepackage{bm} % special 'bold-math' package
\usepackage{comment}
\usepackage{fancyhdr}% http://ctan.org/pkg/{fancyhdr,graphicx,lastpage}
\usepackage{color}
\usepackage{atbegshi}
\usepackage{textcomp}
\usepackage{siunitx}
\usepackage{lipsum}
\usepackage{float}
\usepackage{array}
\usepackage{tabularx} % for 'tabularx' environment
\newcolumntype{C}{>{\centering\arraybackslash}X}
\usepackage{booktabs} % for '\toprule', '\midrule', etc
\usepackage{amsmath} % for '\dfrac' macro
\newcommand \widebox [1]{\setlength\fboxsep{6pt} \boxed { #1}}
\newcommand \ev [1]{\left\langle #1 \right\rangle}
\newcommand \evd {\operatorname{E}\evdel}
\newcommand \bbrac [1] {\left ( #1 \right)}
\newcommand \bl {\biggr | }
\newcommand \espa {\\[2.5 mm]}
\newcommand \vbrac [1] {\left [ \,#1 \,\right]}
\newcommand \dpart [2] {\frac{\partial{#1}}{\partial{#2}}}
\newcommand \kai [2] {\chi_{#1}^{\, (#2)}}
\newcommand \ud {\uparrow \,\downarrow}
\newcommand \du {\downarrow\,\uparrow }
\fancyhf{}
\pagestyle{fancy}
\chead{MODERN \,\,\,PHYSICS\,\,\, LABOTORARY \\[-0.9em]}
\lhead{PHYSICS \textbf{18L} \\[-1.1em]}
\rhead{23 APRIL 2015 \\[-1.1em]}
\cfoot{\thepage}
\AtBeginDocument{%
\rfoot{\raisebox{1.5pt}{\scalebox{0.7}{\textcopyright}} 2015 Physics Department}%
\AtBeginShipoutNext{%
\rfoot{}%
}%
}
\renewcommand{\headrulewidth}{1pt}
mis a tricky beast but you don't need it here as you only have one line in each cell, try withpinstead.